Hey: if you can't make friends, buy them. That's what Barabas does, at least. And he buys a friend—well, slave—who just happens to be his ideal companion.
Barabas couldn't have picked a better Turkish captive. Ithamore hates Christians, has zero problem with deceit and murder and, even better, has zero affiliations or friends in Malta. Before you decide that Barabas just got ridiculously lucky at the slave market, though, let's take a closer look at that first meeting.
We don't know that much about Ithamore or what he wants; he's basically a blank slate, willing to do whatever Barabas wants—he introduces himself by saying "my name's Ithamore, my profession what you please" (2.3.166-7).
But he's not quite a blank slate. He was born in Thrace (Thracians had a rep for being cruel and barbaric) and is totally down when Barabas explains the following pre-reqs for working with him:
First, be thou void of these affections:
Compassion, love, vain hope, and heartless fear.
Be moved at nothing, see thou pity none,
But to thyself smile when the Christians moan. (2.3.170-73)
So basically, be a cruel, murderous psychopath, just like Barabas. And after Barabas spins his own history of stereotypical Jewish evil-doing, Ithamore in turn tells Barabas all about how he's burned down Christian villages, murdered random travellers and crippled religious pilgrims.
So…bad guy, right? Well, yeah, Ithamore's definitely not a moral person, but it's also not 100% clear that this is all him. He tells Barabas he'll do whatever he wants, Barabas tells him "I need you to be a murderous, Christian-hating sociopath," and then Ithamore—presto!—is that thing. In the same way that we're not really sure that Barabas's little speech about his past evil deeds is legit, we're also not sure Ithamore's story is totally true either.
Let's go back to that first line—"my name's Ithamore, my profession what you please" (2.3.166-7). The term "profession" gets tossed around a lot in this play, usually to highlight hypocrisy. Ferneze, for instance, extorts the Jews by "professing" his belief in particular civic and religious principles when really he's just trying to save his own bacon.
The key here is that "profession" doesn't mean career. It means something more like "religious or moral credo"—the thing that you would profess to believe. And Ithamore, unlike anyone else in the paly, has no profession.
That said, you could also look at Ithamore as encapsulating a key theme of the play.Everybody's got an angle, here. The friars want the wealthiest converts, Ferneze wants the Turks off his back, Barabas wants (among other things) to regain his money and power. And everyone is largely willing to espouse whatever religious or legal doctrine is convenient to get those things. Devotion to principles? Nice if you can get it, but certainly not worth stressing about.
So, when Ithamore says, "my profession [is] what you please," you get the feeling that everyone's thinking it, and Ithamore's just saying it. As a slave, Ithamore really embodies the dynamics of society where "everyone's price is written on his back" (2.3.3).
Barabas and Ithamore: match made in … well, somewhere. Right?
Not so fast. The problem is, no matter how loyal or evil Ithamore is, the Barabas Kills Everyone Operation is a one-man show. Once he hears about Abigail's conversion, Barabas immediately disinherits her and tells Ithamore that he's adopting him as his sole heir. But he's lying. Barabas tells the audience in one of his asides that Ithamore will "ne'er be richer than in hope" (3.4.53), meaning that he doesn't really plan on giving Ithamore anything.
Of course, the betrayal isn't one sided. Ithamore is treacherous, too, blackmailing Barabas once he's taken in by Bellamira & Pilia-Borza's House of Sexy Wonders. They may be really good at executing murderous plots, but the Dream Team they are not.