"The end justifies the means" is always a dubious justification. Selling arms to vicious dictatorships like Saudi Arabia is not without humanitarian cost, as the the casualties of the Yemen war demonstrate.
The end justifying the means also needs some justified ends to demonstrate that the means were tolerable. In Saudi's case we've seen little signs of that. It's still one of the world's most repressive police states. It uses Western weapons in wars of aggression. It's happy to murder people in foreign consulates. There doesn't seem any sign of this improving.
Selling arms to Saudi stops a flow of cash to Iran and Russia, but we're just propping up one horrible regime with the supposed excuse of weakening others. This just seems like an excuse to make some cash.
> eventually the US selling weapons to Saudi Arabia will be viewed similarly to the US selling weapons to Germany
And where are the signs that Saudi Arabia is moving towards being a liberal democracy like Germany?