What do you think mercutio thinks of fate or destiny? why?

 Mercutio is a young gun and having fun. He doesn't take anything too seriously, even women. He is young and reckless for the most of it, yet like all young men, there is a budding man underneath. 

Mercutio will not tolerate injustice, is very loyal to his friends and ultimately he delivers "a plague on both your houses" - this pivotal delivery wouldn't come from a fellow who thought that fate and destiny are a lot of cobs wobble or outside of his control. 

He is a young gun and having fun, though he is also a pretty layered character. I think he does believe in fate and destiny to a degree, though like most of us, doesn't spend too much time placing his fate in the hands of the gods, he creates his destiny by creating his destiny - until his fate is taken outside of his control.