I think we need to clarify what a "connotation" is from a "definition".

For example, deciding to run from office when you see your opponents are likely not to win is "opportunistic", but it is neither negative nor positive - it is strategic.

Going to the store and being on the lookout for sales, and stocking up on as many bags of beans for a great discount that you didn't plan on seeing is "opportunistic" - but once again, it is neither negative nor positive.

What does Mat do when he needs money? Despite knowing that it is almost guaranteed that he will win every game, he goes in, cleans people out of their gold, and then leaves when he is tired of it. Given his abilities, which even he is not fully aware of but has noticed and commented on, you might call that "stealing". But the key word is that his opponents can either play or choose not to play - their choice, despite knowing his luck, is the only element that I would say separates this from outright stealing. Even then, though, it is "opportunistic".

It's like that play on words - 'fair is foul, and foul is fair'...it all depends on the situation and the actions. There is no objective standpoint through which the word can lay claim to one form of existence. After all, no matter what evil humans may perform there are unintended consequences, some of which turn out to be good and other outcomes whose desirability depends on whom they affect.

An opportunist is any person who takes advantage of any opportunity, whether they seek them out or they are in the right place at the right time. Whom their decisions affect is what distinguishes the moral connotation of the outcome.