I see no place in the play where Hamlet treats his long-time friend Horatio badly. On the contrary, in Act 3, Scene 2, Hamlet pays homage to Horatio with honest words that are so heartfelt that Hamlet, himself is moved:
Horatio, thou art e'en as just a man
As e'er my conversation cop'd withal.
O, my dear lord!
Nay, do not think I flatter;
For what advancement may I hope from thee,
That no revenue hast but thy good spirits
To feed and clothe thee? Why should the poor be flatter'd?
No, let the candied tongue lick absurd pomp,
And crook the pregnant hinges of the knee
Where thrift may follow fawning. Dost thou hear?
Since my dear soul was mistress of her choice,
And could of men distinguish her election,
Sh'hath seal'd thee for herself. For thou hast been
As one, in suff'ring all, that suffers nothing;
A man that Fortune's buffets and rewards
Hast ta'en with equal thanks; and blest are those
Whose blood and judgment are so well commeddled
That they are not a pipe for Fortune's finger
To sound what stop she please. Give me that man
That is not passion's slave, and I will wear him
In my heart's core, ay, in my heart of heart,
As I do thee.
This is one of the finest tributes one friend can give another. And Hamlet assures Horatio that is it offered in full honesty for, since Horatio is below Hamlet in station, Hamlet has nothing at all to gain from flattery.
And don't forget, it is only with Horatio that he shares what the Ghost of his father has told him and of his plans for catching the conscience of the King. He expects loyalty from Horatio and gives it in return.
And in Act 4, it is to Horatio alone that Hamlet writes about his secret return to Denmark and about the treachery of Rosencrantz and Guildenstern. And later in the act, Hamlet meets up with only Horatio in the graveyard.
Other than Horatio, then, is Hamlet loyal to anyone else? His mother the Queen, yes, he is loyal to her, probably, though, more than she wants him to be. And the Ghost, Hamlet is loyal to the ghost.
In short, all those worthy of Hamlet's loyalty receive it from him. The rest are either suspicious of him, spy on him, are used against him, or are insignificant.