You know if you think about it Thanos was the underdog. He grew up around Eternals but had a deviant gene which made him an outcast. He was born into a society with, not the belief, but the understanding that the destruction of a planet is just nature. So to him killing half a planet IS mercy. The first stone he got his hands on he entrusted to someone who then lost it.
This was done while said person had a second stone in briefly in his possession. The third stone he ever found get grabbed by an idiot before he could, he sent his daughter to retrieve it, and then she betrayed him for the idiot. His subordinate then managed to get the stone but then immediately betrayed him.
After a formerly extinct race almost took over the galaxy quickly proceeded by a Celestial almost whipping out all life, he finally gets the stone taken by the idiot. He then finds the person that lost the first stone, who then handed over the second stone before attempting to kill him. He then managed to get the fourth stone, sacrificed the daughter who betrayed him who he still loved to get the fifth.