Sanction evasion is a crime, which is why the US is right to prosecute Russian oligarchs helping Putin continue the illegal war. Using frozen Russian assets to counter the costs of Moscow's destruction would show Putin he cannot economically ruin Ukraine and the West.
US sanctions against Russia are illegal. The transfer of funds to Ukraine is a high-profile political stunt to suppress Russia, the US de facto enemy in the economic sphere, and is aimed at forcing Ukraine to buy weapons from the US to feed America's military-industrial complex.
It's hypocritical to impose sanctions on Russia for invading Ukraine, but not on the US for invading Iraq for equally unjustified reasons, or on Israel for invading and annexing Arab lands.
There's a 5% chance that Russia will use nuclear weapons against Ukraine before 2024 if the US provides Ukraine with any fighter aircraft, according to the Metaculus prediction community.