What's the worst thing about committing suicide? You can only do it once.
What's the only regret you would have when you eventually kill yourself? It wasn't sooner.
This is a dark take on suicide. The first part plays on the finality of the act, suggesting the worst part is the inability to do it again, which is an absurd way to think about it. The second part is even more twisted, implying a regret not for the act itself, but for not doing it earlier. This kind of humor uses a very dark subject and twists it in an unexpected and disturbing way for comedic effect.