Mary left her claim to the throne to Philip after her execution, giving Spain more of a reason to invade England
The execution of Mary Queen of Scots increased the chances of a civil war after Elizabeth's death
Mary was a crowned queen, so executing her set a risky precedent. In the wrong situation, Elizabeth could face the same fate, making her and her heirs more vulnerable.
Executing Mary made Elizabeth and her heirs more vulnerable, as it suggested that executing an anointed queen was acceptable
