An epitaph is a phrase or statement written in memory of a person who has died. They are often seen engraved on tombstones. They also don't have to rhyme, although I like them the most when they do. More often than not epitaphs are serious, but they can also be funny when they use a hefty dose of irony. The aabb rhyme scheme that you are asking about means that the first two lines need to rhyme and the last two lines need to rhyme. The following are examples of what you could write about those two gang members.
"Here lies Johnny cold and dead
His life extinguished on a hospital bed.
He didn't get to live and grow old
So he told Ponyboy to live and stay gold."
"Dallas Winston was no jewel.
He found it a joy to be so cruel.
Decided to control how he would retire
And went down in a hail of gunfire."