Suicide, it's a suicide
The phrase "suicide, it's a suicide" was first used by rapper KRS-One on the song "Moshitup" (Just-Ice feat. KRS-One) in 1987. It has since entered into hip hop music as a meme, and has been used by artists such as Ice-T, Gravediggaz, Redman, Fabolous, Ras Kass, Jay-Z, Jedi Mind Tricks, Snoop Dogg, Kokayi, Goldie Lookin' Chain, Krayzie Bone, and The Underachievers. It serves as a good example of the intertextual nature of the genre, where quoting older works serves as a form of homage to other artists.
The phrase was first used by Boogie Down Productions's KRS-One in 1987. KRS-One, known also as "the Teacher", was one of the most influential proponents of the early socially and politically conscious East Coast rap. He co-produced Just-Ice's album Kool & Deadly, and contributed vocals on the track "Moshitup". KRS-One is vegetarian, and used the phrase to warn against the dangers of eating pork:
...haven't you seen the way this animal lives
why I tell you, suicide, it's a suicide, biddy-bye-bye
suicide, it's a suicide...
A simple explanation as to why this particular phrase has gained popularity is that it just "sounds cool and dangerous". In addition to this, the line serves as an inside reference to KRS-One, one of the legends of the genre, an effect that only accumulated as other highly respected performers repeated the phrase.
The line made its way across to West Coast rap in 1991, when Ice-T used it in the song "Ricochet" from the film by the same name. Two years later it was repeated on fellow West Coast rapper Snoop Dogg's track "Serial Killa" – from the album Doggystyle – by RBX. It has since been used by – among others – Scarface and Redman in different songs both titled "Suicide", by the group Gravediggaz on the single "1-800 Suicide", and by Nelly on Fat Joe's hit single "Get It Poppin'". Welsh comedy rappers Goldie Lookin Chain, who also quote KRS-One's "Sound of da Police" in "Guns Don't Kill People, Rappers Do," appropriated the phrase for their song "Self Suicide." In 2008, Fabolous used it on "Suicide" from his Gangsta Grillz: There Is No Competition – mixtape. The phrase also entered into mainstream pop music in 2008 when it was used by Jay-Z on the remix of Coldplay's single "Lost!" (titled "Lost+").  Nas also used a variation of the phrase in the song "Nah Mean" on the album Distant Relatives, using the word genocide instead, presumably referring to Darfur. LL Cool J did a song "Homicide" using a variation of this phrase too in his 2000's album G.O.A.T..