Genuinely confused. Did I do something wrong?

I was playing Magneto on the Tokyo 2099 map. My team's Jeff was lagging behind so the enemy Venom (who had been playing very well) dived on him. I saw this so I doubled back to help Jeff with a bubble. We managed to kill the Venom together, but right before he went down our Jeff yelled at me. He said something to the effect of "play the objective, do your job!" I saved his life and we got the kill. Should I not have helped?

Maybe he was just a loudmouth, I don't know. The reason this is confusing me is because, in the same game a different teammate also complained about me. Am I playing Magneto wrong? I thought my job as Magneto was to stand in the middle, use my bubbles and my shield to protect teammates, and use my ult to shut down Punishers and Star Lords. I admittedly don't get that many final hits, but I'm always throwing out bubbles and getting upwards of 10,000+ in blocked damage.

Keep in mind that, other than a week of Overwatch when it launched, I'm completely new to hero shooters.