Character deaths can have a strong impact on a story, but only if they fit the storyline. Needless killing off of characters that do not further the plot breaks down trust with a reader. Readers form a bond with characters, especially main characters. They are emotionally invested and if their death makes no sense? They will feel gipped.
Character deaths are not solutions for:
This is the easy way out and readers will notice.
Growing tired of a character
If you have grown tired of a character then perhaps editing and some character development is in order.
They serve no plot purpose any longer
Everything in the plot should have a purpose even deaths, perhaps you should go revisit your plot.
Without a purpose this emotional impact this death will not have the desired effect.
If you are sure that killing a character is to further the plot, then do it justice. Lazy writing out a character death has zero impact on your reader. In all likelihood, it will be noticed but not in the way you intended. Remember, you owe the reader a well-written death.