Onun knew that the children would not go after him, he therefore spilled his seed on the ground... His actions were bad in the eyes of G-D, he therefore killed him too. (Berashis 38/9-10)

One who purposely causes an erection places himself in excommunication. (Talmod Niddah 13)

Whoever wastes seed, thinks impure thoughts, or does something to cause temptation deserves death. (Talmod Niddah)

For many were the evil of man... they committed every possible act of evil, however theirfate (generation of flood) was not sealed until they threw their blood down on the floor. What is the meaning of this, they would spill their seed on the ground. (Zohar, Berashis)

Spilling seed is more severe then all sins, since he defiles his soul in this world and the world to come and he does not see the glory of the shichenah. (Zohar Vyeshev)

R’ Yochanan spoke up and said all those who waste seed are punishable by death. Rav Ami said it is as if he sheds blood. Rav Ashi says it is as if he worships strange deities. (Medrish Hagdol Vayeshev)

Rav Acha bar Yashya said whoever wastes seed equates himself to an animal. Just like an animal does not care what it does, so to this person randomly commits this sin. Just like an animal is set aside to be slaughtered and does not receive life in the future world so to this person stands to die and does not live in the future world. (Medrish Hagdol Vyashev)

Why does the verse say Tinuf (you should not commit adulteries) which is in plural form, it comes to include those who spill seed. 
(Tanah Divi R’ Yishmoel)

At the first gate stands the angel ASTRKH, under his command are hundreds of thousands of deputies. They are appointed on all those who have wasted seed, who do not get to see the shichinah. These people defiled themselves in this world, therefore after death they are gathered by these angels who impose judgment upon them. 
(Otzar Hmidrashim second chamber)

"Bad in the eyes of G-D” like the sin of Onun, who would spill his seed. 
(Rashi Berashis 38/7)

“and also their children” this was counted as murder, meaning they wasted seed and did not repent. (Rashi Yichezkel 23/37)

This was the flaw of the generation of the flood, they would waste their seed. (Rashi Shabbos 41)

Semen is the energy of the body and the light of the eyes. As long as it is excreted in excess the body withers and life is lost. All those that are saturated in this, old age jumps on them, his strength leaves him, his eyes go weak, bad breath pours out of his mouth. His hair and eyebrows fall out, the hair of his legs and armpits become thick and his teeth fallout. Many additional health problems occur.(Shulcan Aruch 240 Rambom, Dayot)

It is prohibited to emit seed in waste. Those that do are put into excommunication, and it is as if he killed another person.
(Rambam esuray biah 21/18)

One who spills seed, bais din is commanded to place him in cherem. 
(Ramban M.M. E.B. 21/18)

It is prohibited to waste seed. Those that do it, not only is it a serious sin they also place themselves in spiritual excommunication and is equivalent to an act of murder. (Tur A.H. 23/1)

One is not allowed to spill seed. This is the most severe transgression from all sins in the torah. A person is not allowed to purposely cause an erection or bring himself to think impure thoughts. (Shulcan Aruch A.H. 23)

It is prohibited to waste seed . This is the most severe of all sins in the torah. Those who spill seed in waste, not only do they commit a major sin, they also place themselves in excommunication. Referring to them the verse says “your hands became filled with blood”. It is as if he kills a person. Sometimes as a punishment for this, heaven forbid his children die young or end up evil people. This also causes poverty. (Kitzur Shulcan Aruch 151)

Wasting seed is more severe then manslaughter, for here he is spilling his own blood and the blood of his children. In the Zohar parshas vyache it says all evil people can repent except those that commit this sin. In addition a person under the age of mitzvot is not held accountable for his sins except this one, for Er and Onun were underage and got punished. In addition the generation of the flood who were wiped off the face of the earth was only for this sin.
(Sharai Kedusha)

More severe then all sins is one who spills seed, for he spills the blood of his kids for all eternity. In addition all sins can be rectified, but this one has no rectification. (Sharai Kedusha 2/8)

One has to be extremely careful with his bris... He should never cause an improper erection... Be very careful not to damage the attribute of yesod. (Tomar Devorah 8, Ramak)

One who spills seed, his punishment is death from heaven. His punishment is like that of the generation of the flood. This verse refers to those that waste seed “Under the trees do they slaughter their children. (Shari Teshuvah)

One should be careful not to waste seed. (Sefer Hyirah, Rabbanu Yonah)

Know that from all the sins in the torah there is none that create demons like one who spills seed. It comes out, all souls who were to be his children now intermingle with the sitra achara. He takes holiness and turn it to impurity, good into evil (Kaf Hchaim 240)

There is a type of znus that is quite common amongst the young people, they waste their seed. This comes from their ignorance of how damaging this act is. It is an obligation to warn everyone young and old, let them know the severity of these issues until they tremble from fear. Every city should appoint men to watch and teach the people to stay away from this terrible sin.
(Pelah Yoetz ot zayin)

Whoever transgressed this and brought into his mind impure thoughts has sinned terribly, this destroys him from this world. It defiles all 248 limbs and 365 sinews, physical and spiritual. Therefore be extremely careful with this.
(Safer Hamitzvos Hkotzar, Chofetz Chaim)

“Lo Tinof” he should not be Mnof by himself, by spilling seed.
(Kitzur Sefer Charadim, Chai Adom)

Also one who has wasted seed should do the following, he should repent by siting in freezing cold water...(Sefer Chasidim)

When a person spills seed many impure forces attach themselves to the hairs of that region, from there are created evil spirits. They are the ones that are called ‘painful afflictions of man’. After he dies they do not leave him since they are his sons, there is no suffering bigger then this, may the merciful one save us. (Shavet Musar 27)

One who spills seed flaws his own thought process as well as the upper knowledge. (Kitvi Ari)

His prayers are not accepted (Shlah 100)

He loses his ability to understand torah (Kav Hayoshar 68)

He causes the lengthening of exile, plagues and all tragedies. His children will also follow the path of evil. (Mam Loez Vayeshev)

Heaven forbid that one should defile his holy seed in such a manner.
(Ravah Vayeshev)

Spilling seed destroys the world, it is counted as if he kills someone. This is referred to in the verse “one who slaughters his kids”. (SvT Maharil 4)

Those who destroy their seed bring a flood to this world. (SvT Radboz 596/3)

He will be placed in excommunication, like all those who spill their seed.
(SvT Radboz 161/5)

He gives power to the forces of evil and weakens the tribunal on high as is known to those wise of the truth, the masters of kabbalah. (SvT Yavetz 43/1)

Know that a person who spills seed creates spirits of destruction.
(SvT Rav Poalim)

Spilling seed is totally prohibited, there is never a time when it is permitted. (SvT Rav Poalim 2/3)

There are opinions that spilling seed is dioraysah (biblical transgression).
(SvT Siredi Aish)