The word "mankind" doesn't have any capitalization.

Is mankind a collective noun?

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.

What words should you capitalize?

In general, you should capitalize the first word, all nouns, all verbs (even short ones, like is), all adjectives, and all proper nouns. That means you should lowercase articles, conjunctions, and prepositions—however, some style guides say to capitalize conjunctions and prepositions that are longer than five letters.