What is the origin of en vouloir?

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)

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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.