Mankind is usually singular nowadays, but if you go back a century or so it was usually plural.
Neither plurality is or ever was axiomatically "wrong", but we tend to avoid saying, for example, "mankind are my friends" because it conflicts with the current "default" singular. Or we might say "all mankind are", because that muddies things up a bit and makes it more "acceptable".
OP is simply wrong to say mankind are great is "incorrect". Some people won't like that, but it's fine to others. To me, it's a collective noun - which can take either plurality according to context.
It's true people would normally say mankind is careless of the environment. Partly because it accords with our default plurality, but partly because that context suits the implication of a single "controlling mind", whereas with [all] mankind are my friends, we're probably thinking of lots of real friendships with individuals, not one metaphorical friendship with an abstract concept.