Essentially, Biden is very much a 'consensus' leader. In other words, he tends not to make strong deviations from policy unless there is very broad support for them. Supporting Ukraine was pre-existing US policy and popular at the time of the 2022 invasion - so Biden intensifying it wasn't out of character for him.
Opposing Israel, on the other hand, would be contrary to established US policy and something that is not widely supported in the US. So Biden is very unlikely to do anything substantial to restrain Israel, regardless of how horrific the situation gets.