Up to you, but our general hope is that downvotes will serve an informative purpose. For example, if you see an answer that's downvoted, then you know not to trust it straight away. If you see a question that's downvoted, then you know the community didn't find it valuable.
Silver linked the flags v. downvotes page, which is probably the best resource to help you decide when you might downvote. But I'll also speak to some of the points you made.
A mod already saw it and commented but didn't downvote, so it doesn't need downvoting, right?
Don't think it's that deep. If you think it should be downvoted then don't let us stop you.
It already has like 5 upvotes, so a downvote probably wouldn't be noticed and wouldn't say or mean anything.
Fair enough, but I've downvoted answers like that before as correct answers often catch up to the incorrect one when people see what's going on.
It is super old and talks about something nobody really cares about, and nobody has seen it for ages, so a downvote again would be useless.
In general, old questions get more views than new questions (because they've had some time to entrench themselves in google and the like). I downvote old posts often.
It's bad, but also outdated, and I just made a new, better answer, so why would I downvote it
Downvotes can still have an informative purpose in this situation.
It's made by a new user who doesn't know how to answer well, so I feel bad if I downvoted someone when it wasn't really their fault
If the answer is lazy or unresearched, I downvote anyway. We don't want lazy and unresearched posts on the site, people can learn the hard way if they must.
I didn't leave a comment because it would feel dumb to say something like "this answer should provide more detail" if it's 15 years old and completely irrelevant
Fair enough
it's a new user who wrote a bad question but it's closed already
Fair enough, downvoting posts that break rules is a bit pointless since there's no informative purpose anymore. No need to dogpile.
