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In an effort to make my question as short as possible, I abbreviated too much. This is the complete sentence that originated my question:

少年時代から２０年間もずっと過ごしてきて、他人から軽んじられるのに慣れていなかった。

Now you can see what I clipped. I removed important information. This question is answered. My mistake. I answered my own question to clear this up. Sorry I caused so much confusion. But, this was very instructive for me.

ORIGINAL QUESTION

(1) イギリス人に軽{かろ}んじられた。
(2) イギリス人から軽んじられた。

Both appear to be grammatically correct 受{う}け身形{みけい} and also have the same meaning:
"I was looked-down upon by English people."

(3) 虎に食べられた。

Appears to be grammatically correct 受け身形 and mean:
"I was eaten by a tiger."

So, if I can swapped "から" for "に" in #1 and #2, can I do the same with #3 and say:

(4) 虎{とら}から食{た}べられた。

#4 does not sound possible to me (but maybe it is?).
So, might it be that "に" is the standard/natural thing to use in 受け身形?
Sometimes, "から" can be swapped in for "に", but in reality you'd need a native ear to sense when that is appropriate?