I don’t think it’s anti-Torah per se, it’s non-Torah. Compassion is wonderful, but when you put it ahead of other values (e.g. oneg Shabbos)in a way which halachah does not demand or even request, you’re just using an excuse to follow an “ism” rather than the Torah.
I think it runs much deeper than that. When somebody sees themselves as more compassionate than the Torah, they are declaring that the Torah is not the ultimate moral truth, since they are more moral than it (chalilah). It is a complete apikorsus in itself.
Rationalfrummie also is completely correct, in analogizing to Shaul. To take it a step further, I always understood the maiseh with shaul in a similar vein with what I wrote in this post. When a person doesn’t act according to the Torah, it is bad even if currently it is a “chumrah”, because once you’re out of the Torah’s morality, you’re on your own morality, and it is ??? ???? ??????.