If you think it, you'll say it, eventually.
If you say it, you'll do it, eventually.
It may not happen over night; it may take ten years, but thoughts become words become actions, over time. If you read a man's words and they are intemperate don't be surprised by his later actions, or the actions he inspires in his followers.
So how do you NOT think it?
The old way. Put it in a box and put the box on a dusty shelf and when you find yourself going there, don't. Leave the box undisturbed. Never explain it or worry at it, just box it or even double box it and leave it alone. Malign thoughts need attention to flourish. Find other, more outward, things to do, like helping others.
And vicious thoughts, cultivated even a little, wreak all sorts of havoc as they can spread virally, almost by telepathy.
And it's worth thinking about the opposite too. What the right thoughts do.