From novel To Kill A Mockingbird:
I'd soon's kill you as look at you.
What is "soon's" short for? I had found a similar sentence
would as soon do something as look at you
So as soon as can be shortened to soon's as?
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"soon's" is short for "soon as".
"I would as soon as do X as do Y" is an idiom of sorts meaning "I would be just as likely to do X as to do Y". This is usually used to make some humorous comparison between a routine action and some extreme action, like here: "I'd as soon as kill you as look at you", meaning, next time I meet you, I would be just as likely to kill you as to simply look at you. A less dramatic example would be, "I'd as soon as quit as have to work a day like that one again".
It is also said without the second "as": "I would as soon do X as do Y". Like here that would be, "I would as soon kill you as look at you."
Note that "as soon as" also has a more literal meaning: at the instant the event happens. Like, "As soon as Harold arrives, we will begin the meeting." Or, "I'll call you as soon as I arrive in Pittsburgh."
It's a cut-down representation of...
I would as soon [as] kill you as look at you
(i.e. - I would kill you as soon/easily/casually as [I would] look at you1)
...where although the second [as] is grammatically incorrect, it's not uncommon in uneducated and/or dialectal speech. Note that although in "standard" speech it's quite normal to reduce the vowel sound in as to a neutral schwa, it's not normal to delete it completely (so I'd as soon die's diet is never valid).
1 This is the standard "matched pair" construction (OP's "ungrammatical" cited usage has three as's).