Why are Ainz Ooal Gown troops attacking lizardmen? Why are Ainz Ooal Gown troops attacking lizardmen in Overlord II (currently ongoing)? Please explain.
They want to subjugate them

Ainz Ooal Gown is trying to expand their power, so gaining the extra land and bringing the lizard-men under control (or annihilating them) beings a small degree of extra security and power to Ainz Ooal Gown. Admittedly it is a huge amount of effort for essentially no reward (the lizard-men are very noticeably inferior). But it is also being used as an experiment/training exercise for Cocytus.