Or why someone asking for a collateral? i think when someone dont pay back.
btw. why i should destroy my account for 0.5 btc
You can use alt-coins as collateral but you can also use other things like Amazon giftcards etc. Theoretically you can run off with the 0.5 BTC loan (which will render your current account useless), buy another Full Member account for 0.1 BTC and still have 0.4 BTC 'profit'.
That's why people ask for collateral, unless you are a trusted member with an older account.
What also makes this a bit suspicious is that you are offering 0.25 BTC interest for 7 days, which is quite a lot.