Why is it more profitable for a node with more than 51% computation power to abide by the rules in the bitcoin network
An attacker with 51% hash power could perform a double-spend (or several). He could send a large payment to someone and then use his hash power to create a longer chain that spends the same coins but sends them to himself instead, making the first transaction invalid.

It would be against his interest because playing by the rules would net him roughly 1 every 2 block rewards (that is roughly $100,000 every 20 minutes) but if he chose to perform this attack the price of bitcoin would plummet making his reward much less.