What Would Happen if a 300 Km asteroid hits the west coast of Africa?

300Km diameter or circumference?

What's the total mass of the object?

How fast would it be going?

What's the composition of the thing?

What angle do you propose the impact would be?

Once you give us all that, then we can probably calculate that the event would fall into the "bad things" category, at least from the perspective of humans. Cockroaches probably won't care.