I've only played evil on fallout 3 blew up megaton and killed the ghouls for tennpeny but for some reason i didnt finish the game like i feel that its best to play as a good character.

Knowing that my actions give different reactions from just doing good and my actions have effects like new dialogue,relationship changes and a different ending is what makes a evil playthrough for me. Doing something like killing a whole bunch of people and the game barely acknowledging it or not acknowledging it all is disappointing and makes an "evil playthrough" not worth it. You can tell a game is a good RPG game when you're seeing things that's worth doing a new playthrough even on your 1st playthrough.

In Bethesda games, being evil gets awful tedious when you realise certain characters are unkillable, which is often what turns me off been full on evil.

Games like Dragon Age Origins or KOTOR II are far better to play as evil characters because there is greater mobility for your evil actions. Fallout 3 and 4 are far too clean cut as far as being good or bad. Way too many unkillable characters. NV had it a little better, I can really only think of 1 truly unkillable character, Yes-Man, everyone else was fair game. 4 had less killable characters, and becoming a raider warlord was fun, but it runs to the same inevitable thread of monotony that the minuteman option did, which is conquering everything is tedious and nothing truly changes as a result.

For Fallout, I usually try to play neutral. Play as a cold-hearted mercenary as opposed to a madman or open hearted altruist, but even this option in 3 or 4 is very limited. The only real option that makes sense for this route is the railroad (don't care about Synths or saving the wastes at large, just wanna get at the institute) and their storyline doesn't really let me embrace the cold hearted character I want. 3's story is just meh. There is only one option for the endgame, which is help the BoS, in a few varieties. Broken Steel has more choice, but even then, it makes little sense.

It's sad, because Fallout 1 and 2 could allow you to be an absolutely menacing scourge of the wastes.