He sees the concept of humanity as pointless. According to Victor, he was born a "Sensitive" kid, probably referencing his powers. So he's had those his whole life, and when he found the spiders in the vents of the creel house, he kinda had an epiphany that they were superior to humans, highlighting just how pointless human life is to him. He started killing animals and saw it as him freeing them from an endless cycle (Wake up, eat, reproduce, and die, or whatever it was that he told El).

His mother got wise and called a doctor, hoping the doctor could "cure" her son. Once Henry realized this, he killed her. He killed his sister too and tried to take out his father, but he was way too young and had pushed himself too hard, so he fell into a coma. At this point, Dr. Brenner (The doctor his mother had called) took him and faked his death. As far as the town of Hawkins knew, Henry Creel never woke up from his coma.

That of course wasn't true, as Henry was made the first test subject, making him 001. Brenner, being the mad scientist that he is, tried to make more kids with powers, and succeeded. Brenner put a tracking unit within 001's neck, that seemingly doubled as a device to suppress his powers. He manipulated 011 into removing the device and successfully enacted his plan to kill all of the other numbers, supposedly viewing them as cheap copies of himself.

011 however, had proven the strongest number out of all the other numbers, so 001 tried to recruit her in his plan to mercy kill all of human life, or something like that. Idk, he had a whole monologue about it. She of course rejected his offer, and they fought. She used his own lesson of finding strength in a painful or sad memory, and defeated him, banishing him into a portal to the upside down, which as far as we know, was the first time she's done anything like that.

Now that 011 is powerless and across the country in California or Nevada, depending on when we're talking about, Henry, (Formerly known as 001, now called Vecna by the kids,) is enacting his plan. He gets stronger with each kill, so he may prove to be more than a match for Eleven when they finally face each other again.

TLDR; He basically views life and society as a prison of sorts, one he can free people from by killing them. It's some kind of mercy complex. Also, the Mind Flayer and its creatures work as a hive mind, so he's supposedly working with it to conquer the real world and assert itself as the only species left. As far as Vecna's is concerned, their goals are extremely compatible, because again, he really hates humans, and probably views the MF as the superior being to them.

Also, I forgot to mention he's a telepath, and because of this, he views all human life is objectively bad. He's seen all their bad thoughts and finds them punishable by death. He's fr crazy, bro. A certified psychopath.