Throwing someone through a window is not avoiding a fight, it is a fight. Grappling is a special kind of attack.
As for "instakill," that is entirely within your control as the DM, since you get to determine how long of a fall and what sort of surface wait outside a window, and thus how much damage can actually be inflicted by tossing someone out.
Also, I suggest you go right ahead and answer any failed attempt for a PC to throw an NPC or Monster out a window (or to a similarly dangerous fall) with that NPC or Monster taking their next turn to try and throw the PC out the same window.