Animals die in extreme agony when it comes to halal slaughter, and kosher slaughter, because they are not stunned before they have their throats cut. This means that the animals will suffer a lot more than they would with stunned slaughter, because with non-stunned animals they are still conscious, and therefore conscious of the pain being inflicted upon them.
All forms of animal slaughter are barbaric, however, halal and kosher slaughter is even more barbaric - as it causes a lot more (unnecessary pain).
In the United Kingdom animals are legally required to be stunned before the are slaughtered. Except when it comes to so called 'religious slaughter' - namely Islamic and Judaism slaughter. There are two problems with this law. The first is that, instead of making the law universal, to apply to all citizens in the UK, we essentially create laws for different groups of people - this is unfair, the same laws should apply to everyone. The second problem it creates is that we are not putting the welfare of the animal first - with this law, it does not minimize the pain of the animals, it increases it.
There are many organizations which speak out about the cruelty of halal and kosher slaughter. They are; The British Veterinary Association, The EU Food Safety Authority and, the Farm Animal Welfare Committee.