The pro gun-control advocates don't see this as a legitimate justification for gun rights at all. They just see more gun violence, and yet another shooting. They don't see good guys and bad guys. They just see gun guys. To them, the assailant was just another gun guy, and a problem that is internal to the gun culture. The argument that we need more gun people to take out other gun people doesn't make sense to them and they're just worried about getting caught in the gun people's crossfire. Therefore, they see a need for an authority to step in and take the guns away from everyone.
If they were in that church that day, they weren't just horrified by two innocent people being killed, but also by all the people, more than a half-dozen, drawing their guns and pointing their guns, and in their hoplophobia, want all the guns to go away.
To them, they just see a church full of people that appear eager for gun play and it doesn't in any way resolve the nightmare that unfolded.
The pro-gun rights people's answer so far has amounted to, "it could have been worse." Which, from the perspective of someone pro-gun control is interpreted as the gun people saying, "we can make it worse."