Meaning of 匙加減 in this sentence

Question

簡単な匙加減で殺せてしまうほど、衰弱し

How does 簡単な匙加減で exactly work here?

"With a simple amount?"

Answer 1

The word 匙加減 basically refers to "(adjusting) amount (of sugar, salt, etc)", but by extension, it commonly implies someone's "(often empirical, casual and/or unfair) discretion/decision".

俺なんて社長の匙加減ひとつでクビになる。
A person like me can be fired at the drop of a hat by the boss.

そういうのって彼女の匙加減(しだい)ですから。
That's all up to her discretion. / That's how she (casually) handles these things!

So (Aの)簡単な匙加減で(Bを)殺せてしまう means B's life is at A's mercy, and A doesn't need to get serious to kill B at all.