The great theme RPGs are based upon can be called heroic fiction, stories of good guys doing what the see as right often in a world where people have given up and turned their backs on those in trouble. The original Dungeons & Dragons is one type of heroic tale.
Gary Gygax, principal designer of D&D, drew upon his reading in pulp magazines, among many other sorts of media, to inform his work. Subsequent designers largely followed his lead, though some have provided more nuanced work.
Most RPGs are games of adventure. Now it is possible to run an adventure without a villain or villains, but it's not going to be an easy task fpr most people.To get ahold of players' attention most adventures need an antagonist, or villain.
An antagonist is essentially against. A protagonist is essemtially for. An elvish protagonist can be considered a orcish antagonist. An orcish protagonist can be considered an elvish antagonist.
But before we consider what an antagonist is we need to consider what evil is, and how it is experienced in the world.
At its most basic evil can be considered the harming of others to benefit oneself. When a creature can be deemed evil relies a lot on how intelligent an animal is. A dumb animal can't be considered truly evil; a wolf for instance, because they can't understand that they are doing wrong. A sapient such as an orc can be considered evil, because he is able to appreciate how his actions harm others, and how to work to ameliorate them.
This leads to the question, is evil irredemable? Is evil something some persons do because it is their nature and they cannot change it? Or can people change their behavior if you give them the chance and the education.
In Bob Heinlein's Friday the hero, a young lady at the time, is taken aside by her father, informed she is essentially immoral, and told that she needs to learn how to fake morality to avoid detection and destruction by the truly moral. Her first lesson is to follow the 11th Commandment,; Dont Get Caught. She does that then the Mrs. Grundys of the world won't persecute her.
But what about the foolish or untaught? They usually end up in trouble. Where they don't end up running things or acclaimed as heros, (The human capacity to adulate the vile and despicable is amazing.)
All this said, the question of whether one can trust an evil creature depends largely on if they are reliable. Foolish people can not be trusted largely, but this applies regardless of morality. Prudent people can be, even if they have a bent towards abuse and maltreatment. You need to make it clear to the evil person that there are behaviors you just will not tolerate.
(Such as dunking a gnome woman in dog pee because she starched your shorts.)
The loyalty of good creatures is virtually assured, as long as they see you as somebody reliable. The loyalty of evil creatures is assured, s long as they see you as someone firmly in charge. Evil people, especially the social animals, tend to see authorities and authoritarians as people to follow and obey. Orcs like strong, capable leaders. As do kobolds and gnolls among many others. Any sign of weakness or indecision and you are likely to end up in the ranks, if not dead. If you must hire those bug bear pikemen, first make sure they never have cause to doubt your ability or sincerity,
But what if you fail in this?
Make sure your people know you appreciate what they have done for you, and that you have come to rely on them. No need to tell them, demonstrate that you trust them, so long as they demonstrate they can be trusted. Also works for good people, but it's especially useful with evil creatures, because they have a tendency to bond with a strong, domineering leader, and to consider him a worthy person to protect when things go wrong.
In most situations people tend to be conservative. Evil people tend to be strict conservatives because the world is a dangerous place, and the typical evil being sees his society, his people and leaders as being a source of surety and security. It doesn't apply in all cases, because some people exhibit the social acumen of a common slime mold, but for the most part evil beings can actually be trusted so long as you have the wit, the drive, and the strength to earn it.
And don't forget that you can't relax, not really. The evil are supposed to be dangerous, If you let down your guard you are insulting them, saying that they are harmless, inconsequential. Not a good message to send, for they will want to let you know that they are a perilous people. Never give then any indication you disrespect them.
That's are look at evil races, and how they can benefit the PC. Coming, how animals naturally behave, and the most likely reason that basilisk is snuffling around your campsite.