So as Jerome had ‘perfect’ genes and had given up on his own life, he was the perfect person for Vincent to ‘become’. He didn’t want to use his life and as such was happy for Vincent to able to use it. Even though Jerome now had a purpose for his life in helping Vincent become him, he was still depressed about his failure and eventually did kill himself at the end of the movie. He kills himself so that there is no confusion about the two Jerome's.
Jerome was similar to Vincent in his basic appearance. He was of a similar age and also had dark hair. While they looked similar, Jerome was a gay bit taller, and they both looked very different from each other. But as Jerome had been hiding from society, no one had seen him in a while and Vincent could be made to look enough like him to pass.
Morrow ultimately ended his life at the beginning of the film by burning in a garbage incinerator. This was presented rather confusingly because of the constant flashes to Vincent going to space. Morrow wore a medal and could be seen through flames proving this.