The Siberian super-volcano didn't just spew out lava and this hellbrew of halogens; it also chucked out an enormous amount of sulphur dioxide, and more importantly the entire volcano complex erupted through Carboniferous coal measures. If this new research is correct, then the vulcanism couldn't really have been much more destructive, because not only was there a super-volcano polluting the atmosphere with ash, dust and sulphuric acid particles (causing a volcanic winter of epic proportions), but the volcano was also spewing ozone-destroying chlorine gas AND on top of all of that had chucked out enough CO2 into the atmosphere to make the oceans slightly fizzy.
Most of the oxygen in the atmosphere is generated by algae living at the ocean surface. Most ocean-living life breeds via a planktonic stage right after hatching, which tends to live where the food is, right at the ocean surface. Pretty much all of the trilobite species bred that way, and the combination of CO2-poisoned oceans and UV light finished their entire phylum off.
The Deccan traps coincided with the dinosaur-killer asteroid, so it is hard to see which had the greater effect on life on earth. Those events did cause a world-wide wildfire and dust-induced ice age, and there was some effect from the asteroid hitting sulphate minerals and releasing sulphate into the atmosphere, but the KT-boundary didn't see this huge CO2 release, and the oceans were much less heavily affected. The KT-boundary event was also much shorter, only a few tens of thousands of years if that, rather than a million years of volcanic eruption.