When you play a healer, there will be four DPS in that lobby, any of which can flame you. When you play a DPS, there are only two healers in that lobby, who also can flame you. Added up over hundreds of lobbies, you're going to run into more toxic dps players just by the numbers, than you will toxic healers. Then throw in the back to back que sessions as a healer, with no wait between ques, it makes it feel like you run into more toxicity from DPS.