It’s important to bear in mind that this passage falls historically at the end of the period of the judges (Samuel was the last of the judges and the first of the kingdom prophet), so there was still a lot of goofy stuff going on – “In those days there was no king in Israel; everyone did what was right in his own eyes.”
The practice regarding the handling of meat offered for sacrifice that’s described in 1 Samuel 2:12-17 doesn’t cleanly fit with the instructions for the sacrifices given in Leviticus. The only meat sacrifice that would have involved the offerer (rather than the priest) actually cooking (in this case, boiling) the meat of the offering in a pot would have been a peace offering. The peace offerings were the only animal sacrifices in which the offerer and his family partook of a portion of the offering.
The peace offerings were to be communal meals between God and His people – celebrations of blessing and fellowship in which God, the priest, and the offerer (and the offerer’s family) each ate a designated portion of the animal. The first step in the procedure was for the offerer to bring the live animal to the bronze altar at the doorway of the tabernacle, to lay his hands on its head, and to slay it. The priest who received the animal would then sprinkle some of the animal’s blood “around on the altar.” The LORD’s portion, which consisted of the kidneys and lobe of the liver along with the fat that covered the innermost parts of the animal, was offered up in fire to God [Leviticus 3:1-5]. The priest then received his designated portion of the offering, and the offerer took his portion. The offerer was to cook and eat his portion of the peace offering within one to two days after the animal was presented to the priest as a sacrifice at the bronze altar. [Leviticus 7:15-18]
Apparently, the two sons of Eli would send their servants into the homes of the offerers who were cooking their portion of the peace offerings (they’d know exactly who the offerers were because they would have just presented the animal at the bronze altar within the previous 2 days), and the priests’ servants would take a fork and grab from the pot whatever portion they wanted. This in itself was a gross violation of God’s explicit design for the offerings, because the priest’s portion and the offerer’s portion were different parts of the animal. Even if we assume that the offerer took responsibility during this period to prepare the priests’ portion of the peace offerings along with their own, the sin of Hophni and Phineas was that they were thrusting a big fork into the pot and taking for the priest “all that the fork brought up.” In other words, they were indiscriminately STEALING the offerer’s portion along with their own.
But the most serious (indeed fatal) violation by Hophni and Phineas was that they were also demanding that the offerer give them uncooked (raw) portions of meat for the sacrifice that included the fat portions designated only to God. Even when the offerer reminded them of the prohibition against doing so, they persisted in their demand [1 Samuel 2:15-17]. For ANY man to consume the fat portions of ANY animal, or the blood, was punishable by death [Leviticus 7:25; 17:10-14; see Exodus 31:14-15 for evidence that “cut off” meant “killed”].
Hophni and Phineas were depriving the people of God of their portions of the peace offerings, but as if that weren’t enough, they were depriving GOD of HIS portion of the sacrifices as well.