Redirects that are misleading to the reader should be deleted or retargeted.
redirects that fail RPURPOSE should almost always be deleted UNLESS some special exception that follows common sense: common sense trumps RPURPOSE. (IAR)
redirects that are unlikely search terms exist on a spectrum. Very unlikely search terms should be deleted through RPURPOSE, likely search terms should be kept, and borderline cases should be kept too, as potentially helpful.
Don’t worry about performance, but maybe worry about cluttering search results.
Give the reader what they want, not what you want.
Don’t hesitate to disambiguate.
If disambiguation seems like an unhelpful option – ie, if the disambiguation page would be full of section links, borderline cases, and “see also” – then delete!
Targets that don’t tell the reader substantially more than the title of the redirect itself tells them aren’t targets, they’re a last-ditch attempt to avoid deletion.
No longer any mention at target? Then add one back in!
Just because a page mentions the title of a redirect doesn’t mean it’s a good target.
Deletion is preferable to a bad retarget.
"No mention at target" isn’t a good reason for deletion, unless it makes the redirect unhelpful or misleading. Synonyms don't need a mention.
If you see me !voting "weak keep: harmless and unambiguous" or something similar, it means that I think the misspelling isn't particularly likely, but its existence isn't hurting the project, so we may as well keep it.