Should I join IIM Bangalore given that I am already earning 20 lpa (base) at Amazon?

No, I don’t think you should join IIM Bangalore and pursue MBA given that you are already receiving INR 20 LPA (base).

If your base is INR 20 LPA then the package should be around INR 25 LPA. An MBA from IIM Bangalore will also get you placed with a similar package maybe around INR 30 LPA. This does not make much difference considering the high fees you will have to pay. In Fact you will be leaving out the market risk, performance, opportunity costs and the increment that you will probably get in the next two years.

So an MBA from IIM Bangalore does not seem a good option since you are already working at an esteemed organization with an above average salary.