To be fair the "child" was a biologically engineered weapon that had just tried to escape and attack him, and was a strong enough psyker to block his original blast of fire that killed every other Eldar around her.

Not only that but her escape lead to the deaths of likely thousands and one of Vulkan's personal friends.

Don't forget Konrad Curze and the Night Lords were also there and it's stated they were about to move in on the Eldar when Vulkan ended it.

Vulkan may also not see the difference between Eldar at that point, he was the one that finally put an end to the Drukhari raids on Nocturne that took there own children to one of the cruelest fates in 40k

Lastly in the Nocturne culture burning purifies the soul making it actually their most compassionate weapon in their eyes.

So tl;dr needs more fire he's a primarch and built those flame gauntlets himself should be able to BBQ 50 Eldar younglings in a single blast.