Wow, so much downvoting for just stating an obvious thing, that extremism is a bad thing, no matter the ideology behind it.
On topic, I can't talk about 2018 because I dont have hard facts about it, but from 1992 through August 12, 2017 the distribution sets that 92% of killings were extreme islamic related, followed by a 6% of extreme right groups.[0]
Again, I find it very inappropriate to try to classify this by ideology. No extremism is less bad than the next one. No matter if its right or left, religious or any other type (remember rajneeshpuram?). Personally, as a non american I have suffered much more of left-wing terrorism (like bombs on subways; kidnapping, torture and killing), but again and again, it doesnt really matter.
[0]https://www.cato.org/blog/terrorism-deaths-ideology-charlott...