Just to recap: at the end of the story, the Invisible Man is beaten and kicked to death by a crowd and Marvel escapes with all of the Invisible Man's stolen money and scientific notes. So what do you think: is that a happy ending? We're honestly not sure. The Invisible Man is a dangerous and unstable guy. But Shmoop doesn't like to see anyone – as evil as they may be – get beaten to death by a crowd.
We also can't forget that Marvel, who totally abandoned the Invisible Man earlier in the story, gets the fruits of all the mad scientist's labor. Not super fair.
One final point: we can't forget that even though the IM is dead, his ideas are not. Marvel has Griffin's scientific notes, and even though he can't understand them, you never know what might happen. Marvel mentions that he wouldn't be evil if he were invisible, but can we trust him? Can we trust any invisible person? Something to think about.