So, bought sime nice leather sandals 3 months ago. I wore them since, and my feet was sweaty in it a couple of times that I suspect now causes this bad smell. Should I put the thing in the washing machine? :)
No you should not put them in the washing machine.
Use saddle soap. Work it into a lather and apply to the leather, wiping it off as you go. Try not to use a lot of water as you do this. If you don't have saddle soap use something mild like Ivory or Dove.
Warning - leather cleaners can change the appearance of leather.
Also check with the manufacturer. Most manufacturers provide such conservative instructions that they are useless but you might get lucky.
Read the instructions for your washing machine, but normally you can, use wool wash or something similarly gentle. Dry them somewhere well aired.
Now go out and buy another identical pair, and wear them on alternate days, so they can get nice and dry in between.
Another solution would be to put a teabag in each shoe for a night.