These were the thoughts of the Framers
The Constitution was not easily ratified by the former colonies. There was not a universal recognition by all and sundry of it as work of genius. Nay, to the contrary, many saw it as a flawed document and arguments were made for and against it. Some of these arguments were held in constitutional conventions held in the various soon to be states. See, Elliot’s Debates. Some were held in a series of blogpost-like essays published by Madison, Hamilton, and Jay in various newspapers. See, The Federalist Papers. And some were held in the form of letters to the editor, miscellaneous newspaper OpEd pieces and even private correspondence amoung the Founders. See, Farrands Records .
If you want to know what the Founders THOUGHT they were enacting, what they intended, you go to these first sources. In law, there are several rules of statutory construction, one of which is you can go to "legislative history" of a law to see what the people who wrote it and debated it actually thought it meant.
You can do that with the Constitution as well.
In late 1787, there was considerable debate about whether the power of the executive should be vested in one person alone, or in a president who could not act without a majority vote of his cabinet. In response to James Wilson’s Speech at a Public Meeting, October 6, 1787 "Every Thing Which Is Not Given, Is Reserved", Samuel Bryan (blogger name "Centinel") made the following reply in an article appearing in the Freeman’s Journal, 10/24/87:
Such a check [requiring the president to get the consent of the cabinet] upon the chief magistrate would admirably secure the power of pardoning, now proposed to be exercised by the president alone, from abuse. For as it is placed, he may shelter traitors whom he himself or his coadjudicators in the senate, have excited to plot against the liberties of the nation.
Virginia Governor Edmund Randolph in a speech explaining his reasons for not signing the Constitution, December 27, 1787 "I Will, as an Individual Citizen, Accept the Constitution" advocated taking away from the president the power of pardoning for treason "at least before conviction". John Steven, Jr. replied to this speech in the [NY] Daily Advertiser , 1/21/1788, saying that since the pardon power has to be lodged somewhere, why not the presidency?
Alexander Hamilton appeared to believe that the president had only post conviction pardon power.
A President of the Union on the other hand, though he may even pardon for treason, when prosecuted in the ordinary course of law, could shelter no offender in any degree from the effects of impeachment & conviction. Would not the prospect of total indemnification for all preliminary steps [ie, pre-emptive pardon] be a greater temptation to undertake and persevere in an enterprise against public liberty than the mere prospect of exemption from death and confiscation, if the final execution of the design, …, should miscarry?
Federalist Papers LXIX [emphasis added].
Hamilton waxes poetic in his belief that the president will be so constrained by honor and a desire to do mercy so as to avoid even the hint of impropriety in the exercise of the pardon power. "The reflection, that the fate of a fellow creature depended on his sole fiat, would naturally inspire scrupulousness and caution: The dread of being accused of weakness or connivance would beget equal circumspection, though of a different kind." Federalist Papers LXXIV [emphasis in original]. He says that a president would so fear the risking the damnation of his fame for future ages –like General Arnold– that such fear would keep him from abusing the power. Federalist LXIX.
Next up, pardon power, the early years of the republic.