I'm just getting some ideas out here, so if you want to ignore this then go ahead.
I was rewatching Storm the Castle episode for Star vs the Forces of Evil. I never understood why Toffee wanted to destroy the wand, but now I do. He never seemed interested in powers or ruling over something. He was trying to die.
I realized this after he healed himself from Star's attack. With that power he could never die. (Think of Deadpool's powers if it makes more sense to you).You're probably thinking 'nobody wants to die, why would he? He's lucky.' I bet after a while he probably got sick of immortality. Maybe loneness, boredom, people he loves dies, or anything else you can think of. I think that the best answer is that life means nothing if it doesn't end. He probably got his cold attitude from this thought. Maybe that's why he didn't care that he was killing Marco. He probably thought that Marco was lucky.
He probably tried everything to end his life. Then he read or heard that Star's wand being destroyed will probably kill him. So he did everything he could to destroy that wand.