They would arrest him anyway or do you think he wasn't being watched by the Russian FSB?
He wanted as much attention as possible. He can't do anything from Germany and neither from hiding somewhere in Russia. So this was his best shot right now.
I'm sure they all fully expected him to be arrested this fast. I'm no expert on the Russian political scene, but it seems clear that he wanted to embarass Putin and is escalating the situation in the public eye to force Putin to act. Rule by fear starts is harder to maintain when it is put to the test, but this requires brave and committed people.
Maybe "weak" might be better - more about a fear of how others perceive him, than how he himself feels. Not a good look for a dictator with a whole secret service at your disposal, when you can't even make a single activist quietly disppear into obscurity.
Well, he apparently still does.. the Bellingcat report is now Navalny's insurance policy, Putin is now thinking twice before doing anything really bad to Navalny, because it could cause mass protests...
Strangely enough the Russian government still want to appear to have a legitimate democracy, probably caring more about the appearance to the governed rather than to outsiders.
As sibling commenter said, bravery, but also to force Putin's hand. If he can show the voting public that this government is really a total farce, pretending to follow laws but actually just a kleptocracy, it would make kicking them out easier..