Why "He looked at me as if I was a spawn of Satan because I was smoking a cigarette." and not "He looked at me as if I were a spawn of Satan because I was smoking a cigarette."

I would say either if fine (you could say "were" is more correct though). I find the a instead of the more distracting than was/were here

For an extra bonus, spawn is almost exclusively plural, and usually uncountable, so it should be, "**the** spawn of Satan." This is because it originally means the eggs of fish and amphibians, which are a plurality in a jelly and too many to count individually.

Because people don't always bother with the subjunctive.

because some people don't know that the grammar is different when dealing with hypotheticals. and under descriptivism, it's acceptable since comprehension isn't affected.

Most British English speakers haven't been taught that kind of subtlety. There was a long period when any kind of grammar was seen as outdated elitism - except for saying I instead of me, which was 100% required at all times in school...

Were is technically correct but people say both. But more importantly, this phrase always has the spawn not a spawn.

The subjunctive mood has been slowly fading from English for more than a thousand years. It is on the verge of disappearing now because we don't really need it anymore.

In general usage, both are equally correct. According to grammar books, not so much, but actual usage is much more important. Side note: it should be "*the* spawn of Satan" in either version, not "a". I couldn't tell you why that's correct at the moment and I'm too tired to look it up, but the phrase "a spawn of [x]" is incorrect, even when it can be reasonably assumed or inferred that the subject of the phrase is not the *only* spawn of [x].

"Was" is nowadays accepted in place of the past subjunctive "were."

In common speech, people often say "was" instead of "were" - especially "If I was" instead of "If I were". It's not "correct" according to certain rules of grammar, but meh; language evolves. For the purposes of learning English, please try to use the more correct version. Don't emulate "lazy" native-speakers who make "errors". Thoroughly understand the rules first, and *then* you can break them.

It’s a note for English learners that ‘was’ and ‘were’ are much more interchangeable in native use than grammar books would let you know. For learners you should generally stick to the rules as taught, to avoid confusion in what you say, and learning every valid exception would take a lifetime, but be aware you’ll see rules broken all the time. For example, in some dialects, you’ll all but never hear the word ‘were’ used at all.

It should be “if I were the spawn of…”

The second one is always correct in formal written English, as you say. Either are acceptable in less-formal English. “If I was” is probably more common in spoken English by now, and always speaking formally doesn’t sound native.

Spoken language is typically different from written language and this is an example of spoken English. That's how people talk. Doesn't matter if it one is technically right or technically wrong. It's how people talk. 

The second is the subjunctive and the subjunctive is dying. Well, at least in American English. I don't know about elsewhere.

The British threshold for the subjunctive is higher than the American. As the OP notes, this is in a British dictionary.