In Christian culture and secular-ish culture in English-speaking majority-Christian countries, it’s popular to talk about how awful the “Old Testament God” is.
This can amount to casual antisemitism, even if it’s not intended. Because this kind of talk is often a coded way of claiming that Christianity is loving and good, but Judaism is backward and violent.
What Christians call the “Old Testament” is what Jews call “The Bible”. So “The Old Testament God” is the God that Jews believe in. It’s not so cool to claim that Christians believe in a loving God but that Jews believe in a violent and vengeful God. It’s not accurate, and it’s a claim that has been used to justify a lot of horrific violence.
The Old Testament God, according to both Jews and Christians, created the world and gave humanity the Ten Commandments. Christians base a lot of their theology on things found in the OT. Christians do not really reject everything done by the Old Testament God. Denigrating the “Old Testament God”, more often than not, is an implied rejection of Jews.
It’s true that the OT depicts God doing some fairly troubling and violent things. But that’s also true of the Gospels. For instance, the Gospels depict a lake of fire in which certain types of sinners are tortured forever. That doesn’t mean that Christians believe in a bad God who likes torturing people. It means that ancient religious texts are complicated and that it’s up to religious people to interpret them in a way compatible with human dignity and human rights.
Christians who believe that the New Testament is a new revelation are entirely capable of doing this. So are Jews who do not believe this. Members of both faiths can be religious in a respectful and good way.
Most people who invoke these claims about the Old Testament God don’t mean any harm, but it is part of an antisemitic tradition that hurts people. There are other ways of opposing the brutality done in the name of religion. It’s counterproductive to invoke the antisemitic trope of denigrating the Old Testament God.