How can absorbing the life force of human sacrifices to fuel spells decrease your own life span?

Gotta use your lifeforce in order to chain up the other person's lifeforce instead of actually using either to, ya know, live.

 

I mean, there's a couple of different options.

My main thought is to view it like building a fire. Yes you can take the wooden logs off of an extinguished fire but on their own they do nothing. You'd require a spark (the witch's life energy) to actually reignite them (i.e., use them for the spell). Then you just give it a rough ratio, like 80-20 dead life to witch life force to explain why they'd bother going through with the whole procedure.

 

The most simple explanation to this would be to think of each person as a lifeforce battery of sorts. Your own lifeforce growths and evolves within your body, so your body is only prepared to hold so much lifeforce. Trying to squeeze more in puts a strain in your body, and the larger the lifeforce that's absorbed, greater the damage. But instead of just saying that any lifeforce absortion process always shortens the lifespan of a witch, you could play more loose with this:

• Small absortions and good training can reduce the strain/damage and make it tolerable or could even being healed completely.
• It's not only the amount, but also how fast the witch is trying to do the absortion. A veteran/skilled witch will be faster and safer than a rookie.
• There may be tricks to avoid damage like storing the lifeforce in alchemical devices, although there's still a risk for the witch because she has to channel the lifeforce through her (like a copper cable channeling electricity).

On the other hand, it doesn't seem very natural to me that someone is born with a certain amount of mana and that's it. Were not machines, we change over time so that mana amount should be affected too. You could say that people in your worldbuilding are born with a natural potential for a certain maximum amount of mana, in other words, a soft/imprecise mana limit. That maximum can be increased with some sort of magical training or enhanced with alchemical skills/devices, and that with old age that natural mana limit usually decreases.

 

The star that burns twice as bright burns half as long.

If this was a star, and another star was feeding it, it would eventually collect so much energy that it exploes.

Suppose a witch, which can max out at 9000, can only super max to a certain level, such as 15000, and then they explode or the difficulty of containing the extra power starts to exponentially increase, such that the amount of personal life force required to contain 9000 is 0, 10000 is 2, 11000 4, and so on. Eventually the life energy required to store exceeds the mana, and Pop...its all comes exploding out. Maybe different individuals have different tolerances, but it all end the same if one goes down that road.