This is highly anecdotal, but... Behring Breivik allegedly had a great interest in mindfulness meditation.
It is no coincidence that Sila is such a central aspect of buddhism. You can't reach buddhahood without it:
One makes an effort for the abandoning of wrong view & for entering into right view: This is one's right effort. One is mindful to abandon wrong view & to enter & remain in right view: This is one's right mindfulness. Thus these three qualities — right view, right effort, & right mindfulness — run & circle around right view.
Buddha means the person who has already eradicated attachment, aversion and ignorance. Hence there is no evil Buddha's. The Buddhist term for them is the "Mara" the opposite of Buddha. The people who practice mindfulness and concentration without the backing of Noble Eightfold Path is called "wrong mindfulness" or "Mitya Sati and Mitya Samadhi"
I am from the Theravada perspective.
There is only one Buddha who opened the path, everyone else is a follower.
Without the 8 fold path, You can go all the way to 4th Jhana and beyond (which is worthless) if you want. Reaching the 4th you create enormous power and ability to do great things. In the ancient times there has been many great sages who did amazing good things as well as some has destroyed whole cities with the population with their single thoughts(according to the scriptures). Yes, they can be Evil.
However, here is the catch, if you are naturally a bad person you can never reach a Jhana. Because the “mechanism” works in the opposite direction. You need to have a well disciplined and purified inner mind to reach a Jhana.
If you do any evil things after your skills development, it will automatically undo everything what you have achieved. How fast it happens is depending on the level of achievement and severity of the evil deeds.
Almost always, if someone knows the 8 fold path and reaching this Jhanic level, will never miss the chance.
It is like you practice throughout your whole life for Olympic gold medal and you are reaching the finishing line ahead of everyone else. But you turn back a couple of feet before and go shoot your legs.