The dictionary definition is to stop something anything suddenly. That is why we say to kill an engine. Some dangerous machines have "kill switches" a means to stop the machine quickly in an emergency. You will also hear people say "kill the engine" instead of switch off the engine. This definition is translated onto people and living beings in the the phrase "to kill someone". you have suddenly stopped their life, stopped them living ended their breathing, heart beating and brain functioning and the functioning of all other organs. From this we get two different legal terms "killing" and "murder" where murder denotes the deliberate and pre meditated "killing" of someone. It is very common to reverse the meaning of something in slang, idioms and daily expressions. So from the definition above for to kill if we reverse it to mean the opposite it can only be good. We use killing it to replace other daily adjectives that would describe good, very well, exceptional, excellent etc.

Yes, like John says sometimes in slang or very informal English a word can be spoken that is the opposite of its usual meaning. That's the case with 'killing it'. The usual meaning is to cause death, but in slang it *can* mean "He's doing it easy" (opposite of doing it badly - ie dying !). Sometimes it is only context and/or tone of voice that gives you the meaning intended. 'He's killing it' can mean 'He's slaughtering it' (in both the figurative OR literal sense) or 'He's doing great'