I have opinions, of course, but I aspire to have them grounded in scripture. However, you asked, so I'll try to share my take on violence and sin. Keep in mind that I'm not an expert theologian. I'm willing to be wrong on these points if someone can show me how I'm missing some detail or theme in the teaching of the Bible.

Also, since the importance of sin is in the intention behind the thoughts and actions and not in the actions themselves, it's possible for both violent and nonviolent acts to be sinful (a).

To start, most forms of violence are currently illegal, and Christians are supposed to submit to their government, even oppressive ones. There are quite a few references for this (b) (c). Finding more examples isn't difficult if you want more. So for violence to be OK, it has to be legal, at a minimum, which is a reasonably high bar in Western countries.

In the Bible, it is made clear that governments are supposed to enforce justice, including with violence (d). It's not clear if "the sword" must extend all the way to capital punishment, but given the fact that capital punishment was prescribed in the Mosaic Law, it seems to be in the picture in some contexts. That being said, I think people can argue in good faith that modern Western countries are civilized and rich enough to afford every possible chance at rehabilitation. But to insist that capital punishment is necessarily murder is ethnocentric at best and possibly false teaching if someone claims to speak for God.

As far as war, it's clearly not sinful in itself (e). Neither is being a soldier or general. Many (most?) of the judges in Judges were involved in battle in one way or another. As were Abraham and Moses. As was King David (he killed Goliath, remember?). But can countries sin by prosecuting unjust wars? Certainly. Can soldiers sin in their behavior on the battle field? I'm sure it's difficult not to.

Ultimately in any behavior, violent or not, God promises rewards and punishments will be fair at the end of the day (f). Christians are supposed to live without sin, with truth, and in love. Of course they have failed and continue to fail in this. But God promises to make everything right, with his blood if necessary. And according to the Bible, it was necessary, because Jesus came to save the world, not condemn it (g).

Is abortion murder? It's a sin in the cases that it's motivated by self-interest, callousness, or even fear. For a Christian that should be reason enough not to do it. What would the point be in calling it murder? To damn people? We don't have to reach far to find that we are all flawed (h). Lust, lying, hate, despising people, neglecting charity, being unforgiving, envy, greed, addiction, gossiping, cheating, consumerism, coveting... everyone does these things already. There's not much point in trying to rub peoples' noses in how they don't follow God's commandments. They either already know they're not following God or don't care (which is basically the same thing).

(a) https://www.biblegateway.com/passage/?search=Romans+14

(b) https://www.biblegateway.com/passage/?search=Romans+13

(c) https://www.biblegateway.com/passage/?search=1+Peter+2%3A13-...

(d) https://www.biblegateway.com/passage/?search=Romans+13%3A4&v...

(e) https://www.biblegateway.com/passage/?search=Ecclesiastes+3%...

(f) https://www.biblegateway.com/passage/?search=collosians+3%3A...

(g) https://www.biblegateway.com/passage/?search=john+3%3A17&ver...

(h) https://www.biblegateway.com/passage/?search=Matthew%205:21-...