Why do straight males seem so selfish/ mean?

That's the stereotype of straight males, and they're true to a large extent.

But why is it so? Is there a connection between masculinity and selfishness/ meanness? Is masculinity after all a negative, destructive quality?

Definitely not. It has been made into one by the social brand of (fake) masculinity represented by social roles of men.

Natural masculinity has both positive and negative aspects, and it can be just as selfish or mean as femininity but just as selfess and caring too. That is why in the olden days, when social manhood closely reflected natural manhood, masculine, straight males were full of character, propriety, selfless, etc.

However, the social roles of manhood, especially those prevalent in the West --- the heterosexual ones, force men to be selfish and mean towards their own inner-self and towards others as well, in order to compete in the intense anti-man race for (social) manhood. By encouraging, rather forcing men to participate in this race, the FOH train them to be selfish in order to survive. The more selfish you are, the farther you will go in the race. And those who appear at the top, the stereotypes of which straight males are made of, are always the one which are the most selfish -- this selfishness is both inherent -- and a learned thing. While the selfless, honest, courageous, real men, don't make it to the top -- because that means that you will also be honest to yourself -- and then you won't be really all that heterosexual.