Running a campaign where the party is evil throws up all sorts of problems, many of which are ethical. Evil isn't just diametrically opposed to good, its also extremely destructive. As a DM this is very hard to deal with and design a campaign for such that the characters do not destroy everything in the campaign and its also a challenge to set things up so that the players do not desire to wreck everything in the first place. Evil for its own sake is no good; it needs a purpose. See my previous post about running an evil campaign.
Evil also presents a challenge to the players too. Evil clerics may only cast the reverse of their spells that are reversible. So they are denied the use of all healing spells as a consequence and must instead use the spells which cause harm to their opponents. Bless and light are also not granted to them, they may only curse and create areas of darkness, putrify food and drink not purify it and so on.
Not being able to get instant access to healing causes lots of problems for my gaming group. They run from combats which they think may cause them issues and even flee in the middle of combat (well, fighting withdrawals to more tactically advantageous areas or get most of the party out of the dungeon while the highest level character holds the enemy off - this one happened with a fight against lizard folk in a cave). Its quite surprising for me when I know they can win the encounter but they're not so sure and so try and avoid it.
No healing means they have to return to town and pay the local temple, or get some rest, to recharge their hitpoints. In my game, rest recovers 2 hitpoints per day. It's purposefully slow to reflect that injuries do not heal over night. This means the party does tend to rest for a few days after a heavy encounter that has damaged the party (we have eight characters in the party). The party also stocks up on healing potions in towns, although these are quite hard to find in shops, there's an alchemist in one of the villagers who sells them and the thief can tap the black market and his contacts to get hold of them (an ability he gets at third level as a house rule).
There are some more mundane items that the players can use to help with the healing process too. I allow them to buy and apply bandages to a wound. A bandage can be applied after a battle to a wounded character and will recover one hitpoint, applying multiple bandages per battle has no effect. Bandages cost two gold pieces each. There's also a soothing ointment they can pick up from a shop in one of the villages (Oilly Olly's - he sells oil, acid, oil that burns for longer and such things) which they can apply to their injuries before rest to regain one extra hitpoint during the night.
Healing will always be an issue for an evil party but these extra items help alleviate the problem a little without taking away this weakness so feel free to use them in your own campaigns if you wish. Evil parties are supposed to have these problems but giving players some extra options is something they will like.