Curly tells Karen he killed Pinto for the same reason Cain killed Abel, a mess of pottage. Cain killed Abel because his sacrifice to the Lord wasn't acceptable and Abel's was. Curly should have referenced Jacob and Esau, where Jacob trades a bowl of stew for Esau's birthright.
If the Tascosa in this story is the same as present-day Tascosa, TX, it is about 4 hours away by car from Dodge City. So when Festus secretly follows Matt to Tascosa, with both on horseback, it is unlikely that he would have held back for a night or two so that Matt wouldn't know he was going there too. They are fairly close to Tascosa when Matt realizes Festus is there.