Meaning of "murder to resist" in the expression "Power can be murder to resist"?

Solution 1:

"Murder to " do something means that it is very very hard. For example "That mountain is murder to climb", or "That course is murder to get an A in".

In this case the tag is playing on the literal sense of murder also.

Solution 2:

this is idiomatic and it means "Power can be very difficult to resist", which in turn means "When offered power, it can be very difficult to refuse to take it." 'Power' in this context means having influence over someone or something, rather than electrical power etc.

The film also involves the crime of murder, so there is a play on words. Presumable the taking and exercise of power involves murder.