Thoughts can be constructive or destructive. Dwelling on either type leads to attitude then actions. We all have thoughts running through their heads that aren't healthy, but it doesn't mean they belong to us. Our brains reject a good share of them, as they don't fit in with the rest of our thinking process.
One thought, dwelt upon, can change a person over time, because one thought leads to another. We all need to have consistencies with our other thought tendencies, so that we can know who we are and what we're about. If we get too inconsistent, it leads to mental instability, as does bitterness (unforgiveness) and it's roots. Unforgiveness is the single most necessary of all to get rid of, and all humans must take care of that, daily, if necessary, before they can move on, no exceptions. But when that's done, EVerything else is taken care of. No need to worry about "evil" thoughts taking hold.