If you remove the obscure terms you're left with 46 words of which 38 are negative, which is a nastiness quotient of 83%. Just look at all the fun and horrible sentences you can dream up:
The snivelling snitch sneered at the snaggle-toothed sneak.
The snotty snob snubbed the snarling snake.
Other such groups of words have been found (although I believe I'm the first with the SNs*): there are the ump words - dump, clump, frump - and a peculiar number of words referring to dim light that begin GL: glisten, glimmer and gloaming. It's called sound symbolism and academics are forever snarling at each other about it.
There are two main theories on the subject. 1) It's a complete coincidence. There are a lot of words in the language and something like that is bound to happen**. 2) People naturally look for patterns in language. Perhaps there were once only a few nasty words beginning SN, but there were enough that when people started inventing new words or altering old ones we did it to fit the pattern. This effect snowballs and we even start to let the pleasant SN words die out because they don't fit.
I believe in the second theory. The reason? Because in that tiny sample of the English lexicon, in those five pages of a seventeen-hundred-page dictionary, are two words coined by Lewis Carroll: Snark and snicker-snack. The Snark is, of course, the mysterious and deadly centre of The Hunting of the Snark, and snicker-snack is the sound of the Jabberwocky's beheading.
This shows that Carroll, the master word-coiner, could discern the pattern and use it to his advantage. Moreover, when I checked in the OED I found that there were lots of neutral SN words that had disappeared and died. (Although the OED did list the delicious word Snollygoster, which means a shrewd, unprincipled politician).
This rule of the snarling SN also has consequences for the naming of fictional characters. For example, J.K. Rowling was on to a good thing when she called her villain Severus Snape. (Snape does pop up in the OED meaning a rebuke or a change to cold or bad weather, but Rowling named him after a village in Suffolk).
The result of all this is that you can invent a word and drop it into conversation and it will sound right. "That snikey bastard!" you can shout. "I'm going to snobble him."
Of course, it may just be coincidence. It may be that the theory of sound symbolism is a fiction, a fib, a fallacy, a fantasy, a phantasm, a falsehood and a fraud.
But if you wanted clinching proof of SN's hellish and horrible connotations, I need only say that it is also the post code for Swindon.
Finally, and simply because we're here, my three favourite SN words were:
Snicket - A narrow passage or alleyway
Snirt - A suppressed laugh
Snool - One who submits tamely to wrong or oppression
Snudge- EITHER to save in a miserly way OR to be snug and quiet
Supernova SN1987a (I bet it's horrid)
*It has been noted that there are several SNs relating to the mouth and nose (snout and snuffle), but not the negative aspect that I'm pointing out. Mind you, I stand ready to be corrected. There are also a lot of words to do with cutting.
**This is the classical, Saussurian view. Saussure famously said "The sign is arbitrary"; unfortunately he was driving at the time.