Should I tag my posts that include murder or violent crimes(News) as NSFW on Lemmy?

It depends IMO picture or thumbnail with serious gore yes. Simply text or images in article no.

If people don't want to see posts about certain subject matters they can filter out keywords to avoid them.

I was really hoping that Lemmy would separate NSWF/NSFL.

It's time for the internet to make a distinction.

NSFW / 18+ tags are usually used for nudity, sexual content, or gore. So, things you either don't want or can't look at. Using them for just news articles on violent crimes (that don't show graphic images) or even things like swear words is excessive and unnecessary and ultimately degrades the tags meaning.

Only if the headline or image is graphic

If anyone is upset by that type of content then they’ve probably stopped reading the news anyway.

If it’s on a major news site then I don’t think tags are needed.

Ha, I guess I inspired this post.

I usually have "NSFW" post hidden on this account to avoid all the porn on /all, but this is very much the account I want to read the news on.

Proper news outlets will usually have their own NSFW warnings and blurr in place to alert people even after they clicked the link. If your link really contains material that requires an NSFW tag before clicking, it probably doesn't belong in a news community to begin with.

Is the post going to cause someone's coworker to report the person viewing the post? Is the thumbnail or images in post graphic? If yes tag NSFW if no then fuck it.

I'm new here but I would think so

If you are the only one in the entire community doing so, then it wouldn't help all that much - though good on you to be so considerate!:-) But it's not required, and I don't think I'd use the NSFW marker for this, if it were me. You could rather use a hashtag or some such for maximum friendliness? e.g. "[violence]", to allow people to use filtering keywords (for those that use apps that even support that - I don't use apps so I don't know which ones can do that).