Why were some delegates opposed to having a president to lead the executive branch

Delegates at the Constitutional Convention were worried about having an executive branch because they thought a president would act too much like a King.

This fear of having another king was a real threat to citizens when creating the US Constitution. Considering that one of the reasons why America became independent was due to a King's actions, the delegates wanted to make sure that they did not establish a similar set up. This is why the Constitution consists of a system of checks and balances. This means that the power of the president is limited both by the legislative branch (aka Congress) and the executive branch (aka the Supreme Court).