Roses are red, get in the van

Alright listen up, you silly goose. This joke is funny because it starts all nice and poetic with "roses are red," but then it takes a dark turn. "TDM" probably means "To Death Match", like in a video game, and then BAM! Suddenly, there's a gun and a creepy van. It's funny because it's unexpected and a bit scary, like when you accidentally watch a horror movie instead of a cartoon. You probably do that a lot, you dummy.