World must act to prevent ‘ethnic cleansing’ of Gaza, António Guterres warns

No, prevent as in stop Israel from finishing.

Putin received an arrest warrant from the ICC and faces severe sanctions.

Same ICC which took one month to hand Putin his arrest warrant is sitting on Netanyahu's warrant for half a year.

The UN is a diplomatic organization that represents the entire world and exists to provide non-violent outlets for issues between nations. Being present at BRICS, which is rapidly adding membership and represents a counter-bloc to Western oriented alliances like NATO and the G7 (which expelled Russia in 2014.) is part of that mission.

Guterres was at the G7 summit https://www.un.org/sg/en/content/sg/press-encounter/2023-05-21/secretary-generals-press-encounter-g7-summit

Why shouldn't he be at a similar summit representing:

Combined, the BRICS members encompass about 30% of the world's land surface and 45% of world population

The UN isn't (rather: shouldn't be) a Western organization. It's a global organisation. His attendance there is a necessary part of his duties.

From the perspective of any nation on the receiving end of US aggression his participation at the G7 would have been seen as equally upsetting. You're letting your bias in favor of Ukraine color your judgement.

Evidently, he’ll go to an economic conference aimed at boosting Russia’s flailing, war-sanction-crippled economy, instead of a concurrently-scheduled peace summit aimed at finding solutions to Russia’s expansionist war of aggression in Ukraine.