EDIT: I didn't realize that there were typos in the quotation:
The original one should be
臣 以臣弑君、可謂仁乎 呼
I suppose 臣以弑君 might be an uncommon variant, but it is still grammatical.
臣以弑君 = 以臣弑君 killing his lord as a vassal.
以 is used as a preposition “as” but the word order is reversed, making it looks like a post-position. The sentence structure is similar to 以下犯上 and 以弱胜强
You can also think this 以 means “and”, so 臣以弑君 meaning “being a vassal but killing his lord”. Although it's common to see 以 be used as “and”, I cannot think of any examples of using 以 as “but”. When I wrote this, I just thought 以 and 而 are sometimes interchangeable. e.g. 臣而弑君 sounds fine to me, which is similar to “人而不仁”, “人而无信”, “君而知礼” ... But 人以无信, etc. do not sound good.
I think either interpretation doesn't make much difference.
以 originally meant “take” or “use” and was later used as a preposition meaning “by”, “through”, “via”, “with” “because”, etc. e.g.
王以舟去國 (by the way, where does this sentence come from?)
It's also common to put the object before it, e.g.
仁以為己任 = 以仁為己任, take 仁 as one's responsibility
However, 何以 and 是以 almost never become 以何 and 以是.
Sometimes the object 是 (this) or 之 (it) does not appear at all, which makes 以 look like a conjunction meaning “in this way”, “because of this”, “in order to” or “then”.
Sometimes, the original meaning of 以 becomes totally lost and 以 becomes a pure conjunction meaning “and”, which is similar to 而.