Definition of alignment (p 33-34, Player's Basic Rules v 0.2):
Thanks EzekielRaiden!
"
Lawful Good (LG) creatures can be counted on to do the right thing as expected by society."
Copied from PHB p. 105:
"Stories of warlocks binding themselves to fiends are widely known. But many warlocks bind themselves to patrons who are not fiendish. Sometimes a traveller in the wilds comes to a strangely beautiful tower, meets its fey lord or lady, and stumbles into a pact without being fully aware of it."
From p.106: "What kind of relationship do you have with your patron? Is it friendly, antagonistic, uneasy, or romantic?"
A traveller encounters a lord of the Seelie Court. The traveller enters a romantic union, marries the lord, and says "I do", swearing to cherish, honor and obey. The marriage oath, in this case, is also a warlock pact, because the fey lord's magical power rubs off (so to speak) on the mortal. (See also: Polgara and Durnik in the Belgariad.) Having stumbled into this pact, the mortal joins the Seelie Court ("thy people shall be my people", as Ruth said), follows its rules and becomes one of its defenders, possibly risking their life to fend off Unseelie attacks... while also insatiably seeking to know the husband/patron/lord and his ways ever better.
This warlock's story is straight out of the canonical PHB text, and "can be counted on to do the right thing as expected by society". (The warlock's husband/patron is both a maintainer and an arbiter of the expectations of Seelie Court society.) This warlock thus matches the canonical description of Lawful Good, quod erat demonstrandum.
A Great Old One warlock, however, seems more likely to "act as their conscience directs, with little regard for what others expect", and thus be Chaotic Good.
Definition of Necrotic damage (Player's Basic Rules p 75, emphasis in original):
"
Necrotic. Necrotic damage, dealt by certain undead and a spell such as
chill touch, withers matter and even the soul."
I've seen old, sick, withered people. I've seen burn victims. If you'd rather kill people with Fire damage than with Necrotic damage, then you are cruel, and thus evil by my standards, though not by the PHB's standards. If you claim that withering someone to death is more inherently evil than burning them to death, then I don't see how your definition of evil comes from the PHB.
"I'd say you have to be evil, beyond any doubt. The class is more evil than the assassin or the necromancer, as written."
Okay, your turn to find anything in the 5E PHB which supports that assertion. Good luck!