So using your basic example above:
block 1 cast bestial wrath
block 2 cast kill command
block 3 cast barbed shot
every time you press the macro button, it will cycle through those abilities. First press will try bestial wrath. 2 press will try kill command, 3rd will try barbed shot. It won’t wait until that ability can be used, it will just try to use it every time you activate the macro. Unless you are using cast sequence commands (results are extremely varied), it will simply try the ability and then move on to the next one whether it executed it or not.
I know that typically with mine, I will put things that I always want to fire off when available at the very top of the block.
I will give you some advice though. When building a macro, go block by block and test it at a target dummy. Don’t try to write out an entire sequence/rotation all at one go. Start with a few simple things and test it to make sure it works. Then start adding and testing.