Interestingly enough, after have some conversations and reading about this question (that is a valid question, and was downvoted!) I decide to read again the Help from Stackexchange.
It's available here: https://stackoverflow.com/help/privileges/vote-down
And I could rephrase it all, but just quoting it is sufficient (with some bold and italics made by myself):
When should I vote down?
Use your downvotes whenever you encounter an egregiously sloppy, no-effort-expended post, or an answer that is clearly and perhaps dangerously incorrect.
You have a limited number of votes per day, and answer down-votes cost you a tiny bit of reputation on top of that; use them wisely.
What are the alternatives to down-voting?
The up-vote privilege comes first because that's what you should focus on: pushing great content to the top. Down-voting should be reserved for extreme cases. It's not meant as a substitute for communication and editing.
Instead of voting down:
If the post is spammy or offensive, flag it.
If the question is duplicate or off-topic, flag it for moderator attention.
If something is wrong, please leave a comment or edit the post to correct it.