Why should I get a bond with negative interest instead of having a bank deposit account either zero interest or positive interest I don't get why central banks apply negative interest rates. They say that buy our bond and at the maturity, it will worth less than today. What is the policy outcome of such decision?
Why an investor still accepts this or is it just a mechanism to indirectly discourage bond sellings?
It makes little sense to me either, but here are some possible reasons for buying a bond with negative interest rates rather than depositing the same amount in a bank:


The deposit-taking bank may go bankrupt during the lifetime of the bond and your deposit would be too large to be guaranteed, with the cost of default insurance being higher than the the negative interest
The bank may start charging you for holding your deposit at some time during the lifetime of the bond, at a higher cost to you than the negative interest
You may be legally obliged to hold bonds, for example as a pension or insurance company
You run an Exchange Traded Fund or similar investment tracking a bond index and some of the bonds in the index have negative interest rates 
You are a foreign investor who only cares about the combination of the current exchange rate, future exchange rate and interest rate, not any individual component of this triangle, or you believe currency appreciation will more than compensate for negative interest
You are a derivatives trader, and the market prices of derivatives lead to an arbitrage opportunity where you can only guarantee a profit by buying the bond 
You believe that at some time in the future you may be able to sell the bond for more than you paid for it (i.e. an even more negative yield), perhaps to a central bank's quantitative easing programme