1.If you are playing a game with multiple endings, do you go for 100% completion and try to get every ending, including the bad ones?
100% completion. If that thing is listed in gallery or extra, I will get it. If not, then I won't... though that simply because I don't know the existence of those things (like bad endings). In good game, I even simply try all choice combination just to see if there's any changes in dialogues.
2.Do you try to avoid the bad ends? If so, what if the bad ending will give you a new CG - would you go for it then?
Generally, I will avoid bad endings. Since that means game over, right? Or meaning you load previous save and choose some other choices to keep continuing the story. Though the biggest reason is simply because I don't know the existence of bad endings, so I'm not purposely chase after the bad ending.
Special case is Radiant Historia. It's a game with lots of bad ending, and all of them being "official ending" since they are listed as events. And to complete all the events, you have to purposely get the bad ending. And since I am a completionist, I get all those bad endings.
And yeah, if a bad ending gives you a new CG, then I will definitely get it, without fail. I remember I was once stuck in a VN where I can't unlock one (only one left from all those CGs!!!) CG of a particular girl. Then I purposely get a bad ending for that girl (for I forget what the reason is). And I was pretty surprised that that CG is for that girl's bad ending.
3.If you get a bad ending on your first run, does it discourage you from replaying the game?
Nope. But don't continuously bombard with me with one same bad ending simply because the choices are all so hard and so obscure that you will need a FAQ to breeze through the game. That will surely discourages me to finish the game. Though if I have the FAQ, I will finish the game.
4.Do you think bad ends are necessary, or do you hate them?
Of course we need them. It's to tell the player that they have choose bad choice and to tell the concept of consequences. I never hate a game that include some bad endings, but I hate a game with ONLY bad endings.
And any other opinions you have on bad endings will be helpful 
Make a tasteful bad ending (don't just a text message tells you that "you dead" or "you are stupid") and that have impact on story. Also make variety of bad ending that show why player's choice is wrong/bad.