Which ancient cults would cut out the hearts of live animals and offer them as sacrifices? The Mishnah (Avodah Zarah 2:3) mentions a concept called עורות לבובין which Maimonides and other commentators explain refers to the pagan practice of making an incision in live animals and removing their hearts for ritual purposes. Do we have any documentation as to which ancient cults were engaged in this practice?
Likely widespread practice
First, we must establish some basic facts. There is a lot of proof that human sacrifice was widespread in region of Middle East and North Africa. Idea that Carthaginians sacrificed their children is not a Roman slander, there is hard archeological evidence for that. Sacrifice to Moloch and Baal is well known from Bible, but also mentioned in other sources (Greco-Roman) . Even episode of Binding of Isaac is now interpreted as a proof that ancient Jews did offer human sacrifice to their tribal god Yahweh but stopped and replace it at certain time with animal sacrifice. Strong rebuke in Old Testament against human (especially child) sacrifice is repeated several times (for example in Deuteronomy 12:30-31). Such strong rebuke would not be needed if even worshipers of Yahweh didn't indulge in such practice.
What about animal sacrifice ? Episode with Isaac tells us that animal sacrifice was some kind of substitute for human sacrifice. Of course, this was not unique for Jews, about same time all other civilizations in the region started to move away from something that more and more looked barbaric. Animal sacrifice persisted well into Roman times, for example in episode of Cleansing of the Temple, Jesus expels money changers and merchants of livestock ready for sacrifice. This supposedly happens somewhere around 30 A.D. Even if do not believe in story about Jesus, it is certain that writers of Gospels were familiar with the practice.
Question now arises, in a period between abolishing of human sacrifice and destruction of the Temple in 70 A.D. (which most likely ended animal sacrifice), in what manner were animals prepared for slaughter. There is a well known Jewish practice of Kosher Slaughter, or more precisely Shechita. According to Jewish tradition, these rules are ancient, given to Moses on Mount Sinai. However, it is a well known fact that compilation of Mishna happened in first and second century, before that laws about preparation of food and animal sacrifice were oral. Therefore, it safe to assume that existed period before Kosher Slaughter and Shechita were different rules (or no rules at all) existed concerning animal sacrifice. Sacrificing still beating hearts is not that uncommon in human history, Aztec did it on human captives, closer to Middle East there are still similar rituals in India etc ...
What is most likely in our case is that somewhere at the end of human sacrifice, there was a transitional period where animals were more and more offered instead of humans, but rules were not established yet. Therefore, they were often killed in sadistic or at least cruel manner. Again, this was not linked to particular cult, worshipers of Baal, Moloch, Yahweh and any other deity likely copied parts of ritual that were widespread. This is mentioned in the Old Testament, which often forbids exactly that : worship of Yahweh in a manner of other gods. When Judaism started to change from henotheism to strict monotheism all kinds of foreign influences were gradually suppressed, and this includes un-Kosher slaughter of sacrificial animals.