I certainly understand his wariness to make the argument.
Back when I actually had the mental image to argue with people over this nonsense I frequently held my tongue about an argument I really wanted to make:
Even if you could magically make every gun disappear mass killings would still be absurdly easy for anyone with a modicum of intelligence.
Of course that's easy to say, but it's an empty argument if you can't back it up with examples. But beyond pointing out the numerous mass killings done with trucks, can I really provide other examples in good conscience?
My head is brimming with examples of ways to kill many people without firearms that no ban could stop, but the last thing I want to see is to see one of my examples actually used in the real world and wonder if the attacker got the idea from me.