One of his faults was that he didn't kill the cattle, he didn't do soo because some of the people objected and wanted it for themselves, he listened to them and complained to the Prophet Samuel saying soo.
I thought he had made the incorrect calculation that he would offer the cattle as sacrifices to Hashem.
Here is a discussion of the reasons for Sauls failing:
http://ascentofsafed.com/cgi-bin/ascent.cgi?Name=zachor-renaSaul's failings There are different sources enumerating Saul's errors and the reasons for his downfall.
His great humility was misplaced at times and caused him to falter in leadership. When Saul waged battle with the Philistines and Samuel commanded him to wait for him before offering the sacrifices they had prepared, Saul acquiesced to the impatience of his soldiers. He was then told that his kingdom would not last.
He was rebuked for being "small in your own eyes" and not assertive enough in his mission as "the head of the tribes of Israel".
King Saul also thought himself more righteous and compassionate than G-d himself and didn't complete the complete extermination of the nation of Amalek. This was his first historical mission as king of Israel and was meant to usher in the time of the Moshiach:
Instead he spared Agag their king and "the best of the sheep and the cattle, and the fatlings, and on the fattened sheep, and on all that was good; and they did not want to destroy them".
Not only that, but the next morning when Samuel came to rebuke him (after killing Agag himself), initially Saul tried to explain away his actions. Had he quickly realized his mistake and repaired the damage himself, he conceivably could have regained favor in G-d's eyes, for a penitent is even more favorable than a completely righteous person. After being rebuked, Saul did indeed express remorse, but it was too late.
The reaction was quick and harsh. G-d regretted having made him king and declared that his reign wouldn't last long. There are those that maintain that Saul reigned for only 2 years. Also, Samuel ceased to come to him, instead seeking out David, his successor. Regarding this error, the Talmud states, "Be not overly righteous like Saul, who thought to be more righteous [than G-d Himself] and had mercy on the wicked [Amalek]." (Yoma 22b)
All character traits have been created for a purpose and each has its proper function in the world. Both compassion as well as cruelty need be utilized for the service of G-d. It was grave disobedience for Saul to have thought his compassion superior to that of G-d Himself. Yet the decree against Saul wasn't finalized until his error involving the sin of accepting slander regarding the priests (kohanim) of the city of Nob, in which he mistakenly ordered the slaughter of all the priests in the city, of which only one survived.
The Baal Shem Tov teaches us that when one comes before the Heavenly Tribunal upon his death, he is shown someone else who seemingly has sinned and is asked to voice his opinion on the case (just as the prophet Natan asked David to pass judgment on "the poor man's lamb" - see Sam II 12). Only after he passes judgment is it demonstrated to him that his deeds were similar, and thus he decides his own verdict in the Afterlife.
Until this episode, Saul could have maintained that he acted against Amalek according to his overly merciful nature - an error, to be sure. But once he showed his own cruel side regarding the city of Nob, he is judged to have been rebellious against G-d on both occasions.
Anyone who sins and is ashamed, not making excuses for himself, all his sins are forgiven, as the verse states: "'…that you remember and be ashamed, and you will no longer have an excuse by reason of your humiliation, when I forgive you for all that you have done,' says the L-rd G-d." (Ezekiel 16:63). (Berachot 12b) Therefore, David the paramount repentant, merited to have a royal line of kings descend from him, whereas Saul's reign lasted a mere 2 years, notwithstanding his outstanding character traits.