Why didn’t Walt just pay Mike’s guys off?

In S5E08 ('Gliding Over All') Walt makes the decision to kill Mike's guys. However, in the same episode, Walt makes a deal with Lydia to sell his product in the Ukraine. Lydia advises that Walt will clear £2,000,000 a week. That tallies up with the fact that, a few months later, Skyler has amassed about £80,000,000 in cash in the lockup.

Given that Walt could easily have afforded to pay for their silence, why did he arrange to have them killed? Hell, he could've doubled their payments and still turned a ridiculous profit. Yeah, buying their silence was a risk, but surely arranging to have them killed was an even bigger one. If even one of the hits had failed then the DEA would've whisked the surviving witness to a secret location and then it would've been game over. What do you reckon?