I am basing my replies on the assumption that these men were not common mountain bandits but Incas, or followers of the Inca, hired by the High Priest Huascar to capture Zorrino the guide.
Why climb over a sleeping Tintin to abduct Zorrino? And why tie Haddock up and leave him some way further, behind a rock? Remember, the high priest wanted to get Tintin and Haddock killed earlier.
The High Priest, Huascar, changed his mind about killing Tintin when he saw him defend Zorrino the guide against two bullying white men.
The fact that Zorrino, an Indian, was guiding white men to the Temple of the Sun made him a traitor in the eyes of the Incas - note how the Inca King orders him to be killed before Zorrino shows off the medallion.
Huascar may have sent his men to capture Zorrino and take him to the Temple for punishment but also told them to spare Tintin and Haddock's lives. Haddock was bound and gagged because he was keeping guard but Tintin was left alone in order to free him in the morning. The reasoning was that without a guide they would have had no option but to turn round and go home.
Days later, in the snow, the bandits reappear when Haddock goes looking for the llamas and they see him come running down the mountain. But then, miraculously, each one of them have a gun. And, er, how did they survive in the snow and mountains without food?
They may have had llamas of their own at this stage which they left behind while they investigated the avalanche. They may also have depots of food and arms in various locations. If some of the Incas frequently travelled between the Temple and towns and cities nearer the coast then it would make sense to have depots for supplies or emergencies.
They may also have been helped by local Indians. Zorrino would have avoided going anywhere near settlements or villages for his own safety as well as that of his white friends.
it's also a bit strange that they now would want to shoot Haddock. If they had guns, why not attack the three adventurers the same way they did in the beginning when Zorrino was abducted?
After the humiliating way Tintin treated them when rescuing Zorrino, they may have decided that this was now a personal matter. Besides, Huascar's orders may have been more for the benefit of Tintin rather than Haddock.
When they catch up with them after the avalanche they are still some distance away. Tintin and Zorrino could have avoided bullets fired at a long range so they make their way closer to them when Haddock comes running down.