"What does "as many as you can" mean?"
Well, if you sent people into places that were crowded with civilians to blow themselves up, and if you chose those locations BECAUSE there were a lot of civilians there, that would be an example of trying to kill as many civilians as you could. Similarly, if you sent soldiers out with instructions to shoot civilians regardless of whether they were attacking or threatening your forces, that would also be an example of trying to kill as many civilians as you could.
"Why you don't have a problem to a methodical killing of a few each day with 10x the total at the end, but you have a problem with many at one time but 1/10th the total at the end."
I have a problem with both.
"Is it because it doesn't hit you all at once?"
It's because I haven't seen substantiation of that 10X number from a source that I know to be reliable and neutral (or for that matter from ANY source). I also have not seen confirmation that it is methodical on the part of the Israelis.