One, you fight with some modicum of honour and he does not. He would be first to use an unforgivable curse on you. Best avoided, because at best you'd be tortured and at worse killed.
Two, if you beat him (which I fully believe you could) there would be worse retribution further down the line. He's clearly Yaxley's new favourite, which isn't exactly an enviable position but means bad news for the person who offs him.
Three, related to two, being Yaxley's new least favourite (now that Jasper's been sacked) doesn't seem like a very enviable position.
Four, on a very selfish note, I'm really sick of bloody (literally) crime scenes and already have a funeral to attend shortly.
Five, I'd miss you when you're gone.
Six, you should practice what you preach and try to not let emotions rule you.
Seven, I went to the trouble of writing this list to convince you not to.