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.
Okay, is it bad that I laughed when I read your controversial question? : ) It can be proven either way, you realize. I adore Hamlet (especially when Mel Gibson is playing him), but "loyal," he is not. (Then again, no one is loyal to him either. Tit for tat, eh?) There are many examples behind my theory (in that I'm leaving out important characters like Gertrude and even Polonius), but here are my main points: 1. Hamlet can never be considered loyal to Ophelia because he plays sinister mind games with her (no matter if Hamlet is considered sane or not). 2. Hamlet can never be considered loyal to either Rosencrantz or Guildenstern because Hamlet most definitely arranges their deaths.
Hamlet denies ever having given her "rememberances" to cement their love. ["No, not I, I never gave you aught" (3.1.95).] He most certainly DID. What a jerk! Then Hamlet flings insults at Ophelia. "Ha, ha! Are you honest? . . . Are you fair?" (3.1.103). Insulting a woman's virginity as a maiden? Hmmmm, not loyal at all. Not as a lover or even a friend. Then there's the lying: "You should not have believed me, . . . for I loved you not" (3.1.117-118). And, of course, there is the ultimate insult, "Get thee to a nunnery" (3.1.121). I suppose I could ask if Ophelia herself was being loyal when she agreed to spy for Polonius and Claudius, but she's not the character in question, is she?
Next Rosencrantz and Guildenstern:
Claudius plots Hamlet's death, by sending him ahead to England.
The present death of Hamlet. Do it, England, / For like the hectic in my blood he rages, / And thou must cure me. (4.3.65-67)
Hamlet turns the tables on him, sends Rosencrantz and Guildenstern on ahead and has them killed in his stead. This he cryptically writes to the king:
I shall (first asking your pardon therunto) recount the occasion of my sudden and more strange return, . . . alone. (4.7.45-47,52)
Smart? Yes. Loyal? No. Hamlet doesn't like spies, . . . whether they be girlfriends or servants. Long story short: in an effort to remain loyal to his own father, Hamlet betrays them all.
In reading through the play I can not see where Hamlet has loyalty to anyone. He talks of avenging his father's death, but does nothing. Hamlet is an angry, indulged, manic, depressed adult who treats Horatio badly. He loses all his friendships but Horatio, but he still does not demonstrate any real loyalty to him. However, he does entrust Horatio with the job of telling his tale after he dies to others. Some critiques say this is loyalty on Hamlet's part and some say he does this because he loves Hortatio. I think that maybe he just recognizes that Horatio is so loyal to him that he will tell the truth as seen by Hamlet's perspective.