I suspected as much, but figured it wasn't my place to say so... honestly, why do people do this sort of thing? What do they gain out of it? 
You may DDoS and ask for ransom.You can DDoS your competitors website to get their customers.
You can DDoS a Tor node to try to find it.
You can DDoS a service you hate.
Lots of reasons. Internet is not a friendly place.
If i'm mixing .512 btc, and i would receive .05 the remaining .012 BTC will be donated?
If you send 0.512 BTC (512 mBTC) to deposit address, you will receive 512 mBTC chip. You can split it into smaller ones and donate as much as you want.Whatever that caused bitmixer to close(I forgot what caused them to close), might can also be the cause ChipMixer to close in the future if it was something involving the government or something.
Nobody knows what caused them do close. There is only speculation for example here: https://bitcointalk.org/index.php?topic=2194145.20