First, en vouloir has not only the meaning to blame someone :
- Avoir contre quelqu’un un sentiment de malveillance. (To have against someone the will to harm)
- Attaquer, tenter de nuire à quelqu’un. (To want to attack or to be detrimental to)
- Avoir une rancune vis-à-vis de quelqu’un. (To have hard feelings or grudge)
- Avoir quelque prétention sur une personne, sur une chose, en avoir quelque désir. (To desire)
- Avoir du regret, du repentir. (To regret)
And it can also means
Être très déterminé, très motivé. (To be motivated, determinated)
To help you to understand, you have to place yourself in situation:
Something happened, probably caused by someone and you :
- want revenge / to ask for compensation (3 firsts meanings);
- want to have / get it back (to desire);
- want to have done differently (to regret).
→ You want to have something from this person. (excuses, goods, feelings)
→ You are refusing the current situation and want reality to change.
You can look at the reverse translation to have more traductions.