I'm Grace Note. I'm a Community Manager for Stack Exchange.
At the end of the day, a vote represents the opinion of a user, to identify whether a question is useful or not useful in addition to points about clarity and research and whatnot. Voting is the tool for the community as a whole to judge posts. It is as much to identify when a question is poorly written as it is to identify that the problem being asked about... really isn't worth anyone's time. There are ultimately three states to a vote, though, and there's always the option to simply "not vote" when you think a question is not very helpful in your own analysis but figure that others may find it worthwhile or helpful when pursuing that route. Basically, if a post qualifies as "not interesting enough to upvote", you're valid to not vote on it instead to express the lack of interest, and a downvote is unnecessary unless you feel it is a negative to the site.
You're not downvoting just to express your opinion. You're downvoting to express your opinion on the validity of a topic to the community. Taking a personal voting grudge on, say, china here because you loathe the country and everyone in it, that would be petty. You may still vote as such, as it is your opinion to give, but it's not a very healthy way to go about voting.
But if there was a subject matter on which you feel that dealing with questions about them is not useful to anyone in the community, then from that you could downvote against questions of that subject matter. This would be because, moreso than your personal hatred for the subject matter, you're voting because you don't believe the question to be valuable to anyone, which is inline with how one should be voting at all times. It shouldn't be "I don't like this", it should be "This isn't useful to the site". Typically, subjects that would get identified in this way usually end up being the subject of topicality discussions - if there's a community consensus that a particular subject is toxic to the site, then it makes sense to bar the subject.
I'm not fond of murder and the like either, though I personally stay out of news often just to not dwell on it. So I can understand your dislike of seeing those kinds of questions. However, I don't personally think it's a subject matter that's harmful to the community as a whole to discuss, so I don't feel that downvoting everything is healthy, neither to the community nor to you as a voter. There is a more applicable course of action for you, though - ignored tags. In your profile, there's a tab in the top right that says 'prefs', which you may use to identify one or more tags as "ignored tags". These will be greyed out when they show up or, if you mark the appropriately labelled "hide ignored tags" checkbox on the same page, hidden altogether from your view as you browse. Using this would let you navigate the site entirely without needing to acknowledge this subject matter, while not inhibiting the abilities of other users to partake in their own millings-about with the topic. It would also avoid the otherwise necessary viewing of such questions to begin with that downvoting would require.