Fad sayings have been going on since the beginning of time.
Hey we used to say Groovy man.
another one that gets me is No worries.
How about the saying break a leg.
One I hear lately is that is sick, meaning they really like it.
do any of them make sense well to the people saying them it does.

“Break a Leg” actually does have an original meaning, but that’s not relevant to this topic, so I won’t go into it.

I’ve heard comedians say, “I really killed it tonight” meaning had a success but I haven’t heard it used in other contexts.

I wouldn’t say it to the executioner at the Utah State Prison.

My take is that it is slang used by a particular age group. Old fogies like me wouldn’t say “Kill it tonight” – we’d say “good luck’ or ‘Go for it’. But I wouldn’t expect a 20 year old to say it the same way.

The traditional abbreviation for a strikeout. A “backwards K” is often used to denote a called strikeout. Invented by Henry Chadwick by taking the “most prominent” letter of “struck” and reinforced by inference of “knockout” or “K.O.” That connotation still exists, when the announcer says the pitcher “punched out” the batter, a play on words that also refers to “punching” a time clock and the punching motion that the home plate umpire usually makes on a called third strike

I just read a FB headline that said “Bernie Sanders killed it on Fox” so I think you would be fine using the phrase that way.

Literally, no. Figuratively today, yes, in some cultures.

If you’re hip it does.

It makes about as much sense as being “down” for something and “up” for something having the same meaning. But yes, this expression is part of my millennial slang vocabulary and makes perfect sense to me. It can be confusing because “killing” is still negative in a phrase like “killing the vibe”. But referring to a test or a performance or some other kind of achievement, “killing it” is good.

If the person is going to be performing it makes sense, especially comedians. I think you can use it in other contexts.

I use to work with someone who would say “it was the bomb” when something was great, but now I wouldn’t use the word bomb. I don’t want to be in the habit and have it slip in a circumstance where people could over react. I don’t need to be pulled of a plane or something ridiculous. Probably kill it could have the same problem. You never know who is half listening and ready to over react.

If they do poorly, they say I died out there. So, it is logical if they do well, they killed it, whatever it is.