The Luddites were just delaying the inevitable by sabotaging the machines. Even if they were able to hold of progress for a while, they would eventually be unsuccessful. They would be better off trying to find a way to improve their conditions and keep the machines.
Some of their actions were absolutely unjustified while others were more ambiguous. Their violence against human beings was unjustifiable. Threatening people and even killing them for economic motives is not justifiable. Their vandalism against property is a bit more ambiguous. It is certainly easy to see why they would have wanted to destroy the machines that were destroying their livelihoods. On the other hand, destroying other people's property doesn't seem right either. It is hard to argue that the mill owners had a moral right to continue to employ the Luddites when the machines were so much more efficient and productive. If the owners weren't doing anything immoral, why would it be acceptable to destroy their property?