Im sorry to say it but, yes, Achilleus was a murderous maniac.
After the death of his dear companion, Patroklos, Achilleus swore a mighty oath of vengeance and bloodshed and, by Zeus, he was as good as his word. Achilleus beheaded twelve Trojan youths and threw their bodies on the pyre of Patroklos. The following morning Achilleus, running on pure hatred, tore into the Trojans like a scythe. He cut a bloody swath through the Trojan defenses and finally came to the river Skamandros. The river rose up and pleaded with Achilleus not to throw more dead Trojans in his, already clogged, waters. Achilleus cared not, he raised his spear and, in his arrogance, attacked Skamandros as if he were a mortal. If Hera hadnt stepped in, Achilleus would have met his doom against the wrath of mighty Skamandros.
After Hector had been killed in battle, Achilleus tied his body to the back of his chariot and dragged it around the walls of Troy. This was all fitting and proper for those brutal times but Achilleus went one step further. Each morning at dawn, Achilleus would again tie the body to his chariot and, screaming to the heavens, drag poor Hectors body round and round the funeral mound of Patroklos.
As for answers 1 & 3... Humble? I think not. His mother was a goddess and his father was a king! What did he have to be humble about? His mother, Thetis, told Achilleus his future would take one of two paths: he would either, a) leave the war in Troy and return home for a long and happy life, or b) die at Troy and be remembered as a hero forever. As for the Trojan Horse, Achilleus did not live long enough to see the Trojan Horse or the fall of Troy.
These fun facts were taken from The Iliad