requesting good fun strategic team based FPS

For the record, I own TF2, Played it and loved it. Heard great things about l4d, love valve, yadda yadda yadda. My parents seem to have something against valve games. Other games, if i tell them it requires team work, OK GO FOR IT, TF2? "NOES ITS GONNA MAKE YOU A KILLER IT MAKES KILLING PEOPLE LOOKY CARTOONY AND FUN AND OK....whenever you play it I SWEAR YOU ACT CRAZY" ok, ima play l4d "NOES ITS WORSE THAN TF2 ITLL MAKE YOU A KILLORZ" but you dont even kill people" ZOMBIES ARE PEOPLE, THEY JUST MAKE THEM LOOK DIFFERENT AND SAY OH ITS NOT PEOPLE SO ITS OK BUT ITS JUST AS BAD"

excuse me while i make a giant f5 and f7