[Zigûr]

Originally Posted by Galadriel55

why don't you see The Sil for the interesting info that immediately after the War of Wrath Sauron truly repented of his evil deeds and service to Melkor, and he genuinely begged for his own pardon. Only after a while did he return back to his old ways.

You have rather beaten me to the punch there; here's the Professor's remark on the subject from "Of the Rings of Power and the Third Age":

When Thangorodrim was broken and Morgoth overthrown, Sauron put on his fair hue again and did obeisance to Eönwë, the herald of Manwë, and abjured all his evil deeds. And some hold that this was not at first falsely done, but that Sauron in truth repented, if only out of fear, being dismayed by the fall of Morgoth and the great wrath of the Lords of the West. But it was not within the power of Eönwë to pardon those of his own order, and he commanded Sauron to return to Aman and there receive the judgement of Manwë. Then Sauron was ashamed, and he was unwilling to return in humiliation and to receive from the Valar a sentence, it might be, of long servitude in proof of his good faith; for under Morgoth his power had been great.

What we can see from this is that Sauron's refusal to return to Valinor was not because he wanted to "Stay in middle earth and keep bothering people" but because he was too used to being second banana of a powerful realm to go from a position of high status to what would probably have been the absolute lowest rung on the ladder in Valinor.
Sauron did not go down the path of evil for the sake of evil, but out of a love of order (quoted earlier) which was repeatedly perverted to a lust for power. Professor Tolkien also makes this comment in the letter which is included in the Preface to the Second Edition of The Silmarillion:

Very slowly, beginning with fair motives: the reorganising and rehabilitation of the ruin of Middle-earth, 'neglected by the gods', he becomes a reincarnation of Evil, and a thing lusting for complete power

So ultimately he had a second fall: repenting of evil he fell back into evil but in the pursuit of a notionally 'noble' cause: the ordering of Middle-earth. It was not an immediate continuation of his activities as Morgoth's lieutenant.

not being loyal to Morgoth is like being good...it is that black and white. Because morgoth is evil personified, everything bad and evil in the world comes from him alone

This is not completely true. There is a difference between being evil, which indeed derives from Morgoth, and pursuing the same goals as Morgoth, which Sauron patently did not do, given that he was intent on rebuilding and ordering Middle-earth. This contrasts greatly with Morgoth's nihilistic tendencies at the end of the First Age, the quotes for which I now realise haven't been included in this thread so far but which I present (abridged) for the sake of completeness, with my apologies for asserting them earlier. They derive, again, from Morgoth's Ring.

His sole ultimate object was their destruction... This was sheer nihilism, and negation its one ultimate object: Morgoth would no doubt, if he had been victorious, have ultimately destroyed even his own ‘creatures’, such as the Orcs, when they had served his sole purpose in using them: the destruction of Elves and Men... even left alone he could only have gone raging on till all was leveled again into a formless chaos.

And lastly, and most importantly:

Morgoth had no ‘plan’: unless destruction and reduction to nil of a world in which he had only a share can be called a ‘plan’.

In contrast to this, the Professor notes:

Sauron had never reached this stage of nihilistic madness. He did not object to the existence of the world, so long as he could do what he liked with it. He still had the relics of positive purposes, that descended from the good of the nature in which he began.

I think we can perceive from this that certainly by the end of the First Age those qualities of Morgoth's which had first bought Sauron's loyalty had long degenerated. This quote about Sauron from the same text is also worth consideration:

He probably deluded himself with the notion that the Valar (including Melkor) having failed, Eru had simply abandoned Eä

So it would seem that Sauron considered Morgoth a failure; he had failed in the purpose which had first attracted Sauron - the masterful ordering of the world. Sauron was now going to fulfil the task which Morgoth had lacked the strength of character to accomplish. Did he perhaps even consider himself to be better, in that sense, than Morgoth? Possibly another question worth contemplating.