Why can't I beat Snowy Granius during the quest Antabolis Informant?

Well, he is a fairly "small fish". On the wiki page you provided you can see that he is only lvl3 with 54hp. That tells me you are probably not even lvl5 yet. Go around, do some stuff, level up a bit and then come back at him. Or alternatively, steal/buy a destruction scroll (spell) somewhere, and cast that at him. Make sure the scroll does more damage than he has HP (some kind of fire touch would probably be the best way to go). You could also drink some potions that would make you stronger in fight. + Get better equipment. It's fairly simple, you just need to get stronger.

PS - Morrowind actually becomes very easy after some time because enemies strength does not scale with your strength so what happens is that you reach a point when nobody can kill you any more.

As this is the first quest on the main quest line, it's one people are going to run into early. There's a few options; you can go around the long way, and approach the dungeon from the other side; alternatively, you can ambush him. You can cross the bridge by jumping onto the edge of the bridge, which will allow you to sneak across without him noticing when it's dark. Unfortunately, you can't get off without alerting him, and in straight combat you'll die. What you can do, though, is get his attention, and stab him through the railing until he dies.

Morrowind is a game of options. There's always something else to try.