magic the gathering – Is it against the law if the goal is not legitimate?

Sure

Targets are chosen when a spell or potential goes on the stack. Magda’s potential goes on the stack as quickly because it’s triggered, i.e. when the goal of Lightning Bolt is chosen upon casting. Her potential is triggered and on the stack even earlier than the focused creature can change into hexproof. The creature changing into hexproof is not going to take away Magda’s potential from the stack, nor forestall the focusing on which already occurred. It simply turns the focused creature into an invalid goal.

115.1. Some spells and talents require their controller to decide on a number of targets for them. The targets are object(s) and/or
participant(s) the spell or potential will have an effect on. These targets are declared
as a part of the method of placing the spell or potential on the stack.
The targets can’t be modified besides by one other spell or potential that
explicitly says it may achieve this.

The stack and determination will look one thing like one of many beneath – issues can resolve in barely totally different orders, however the end result is mainly the identical barring different results:

1. Lightning Bolt is forged and goes on the stack. Magda’s potential triggers and goes on the stack. Hexproof spell/potential is placed on the stack. Hexproof resolves. Magda’s potential resolves and creates a treasure. Lightning Bolt fails to resolve with an invalid goal.

2. Lightning Bolt is forged and goes on the stack. Magda’s potential triggers and goes on the stack. Magda’s potential resolves and creates a treasure. Hexproof spell/potential is placed on the stack. Hexproof resolves. Lightning Bolt fails to resolve with an invalid goal.