You should probably be more specific about how it was "going out of their way" to ks. Ex: you had 1 auto left on enemy and were clearly going to land it, but your support flash ignite autos to steal it. That would be ks of the bad kind. Or, you were dueling lvl 3 in your bot lane 2v2 and both adc's were about to die. You probably could have landed killing auto on enemy adc, but your support landed a stun/flay/hook and auto to finish off the adc before you landed your final spell/auto. This is not ks. Similarly, enemy bot is fleeing to tower with 1 hit from dead. You are in range to flash auto and your support is too. You both flash auto, but support lands auto first and gets the kill. This is not ks. You should actually be happy with the support in those situations -- they were trying to save your life in "ks scenario 2" and they were trying to secure the kill while saving you from having to burn flash (which you may need to escape a gank later) by flashing to get the kill so you could still keep flash up. Thus in the 2nd and 3rd scenarios, the support is actually being a good support.
Also, you have to realize depending on your elo/rank that most supports are treated like shit or at best ignored and unrecognized (when they play amazing and get their teammates fed or save their asses at cost of the support's life) most games. And if they are not ignored for that, they are flamed. Ex: jungler or mid makes stupid invade... support sacrifices him/herself to save them; support called noob/idiot feeder. Support expertly harasses enemy bot and sets up nice engage onto them 2v2 and lands his adc a kill where all adc had to do was land a point-n-click spell and a few autos. Support does this a few times and adc starts thinking they are a god while never thanking their support. Support pings fed enemy mid or jungle is coming to gank after they are both half hp pushing enemy tower due to winning said 2v2. Either: support backs off while adc ignores and dies, then flames support; or support stays, saves stupid adc who refused to back to safety in time and support dies to save stupid adc and support gets flamed. When supports get games like that all the time, yeah many of them will ks even when they wouldn't have to. Put yourself in their tier 2 mobi boots for a bit and try to imagine life from their perspective.
