For a while (First maybe uh... 14 months of League [joined Feb 2014]), I would not kill AFKs/DCs/Laggers, because I felt like a jerk doing it, since it is kind of unfair. Sometimes, you have 5/5 internet signal, you're sitting right next to your internet router, yet you still have 600 ping, because of stupid Rito. **However, in my last several months of League, I started to have lots of lag issues, and it seemed like as soon as I typed the word, "Lag," in all chat, I would get instafocused by the enemy. And it didn't matter which champ I was playing either. I could be playing the Tankiest Shen, J4, or Blitz late game. Then I type, "Lag," in all chat and the enemy focuses me, the laggy tank, in the 50th minute, taking advantage of me being unable to grab, taunt, or knockup. One time, my PC crashed twice in 4 minutes when I was playing J4 and I was instafocused by the enemy in fights. Oh boy did I make them pay for that late game. **Now I just kill them every chance I get, since that is what I get from 90% of this community. Although I did have a couple of enemy top laners who agreed to farm all throughout laning because I was lagging.
Also, your brother sounds a lot like mine. My brother playing Normal Blinds, playing Master Yi, enemy also had a Master Yi, but he was DC the first 32 minutes of the game. As soon as enemy Yi showed his face, my brother killed him. He would sit at all his jungle camps. Level 12 vs Level 2 in game. He got a good 6 or 7 kills from doing that.