Shield is usually on left hand is because most people are right handed as the statistical majority. Thus right hand weapon, left hand shield. Yeah sure we had all sorts of superstition with left hand before so we tried all sorts of ways to "correct" left handers, but it still wouldn't change the fact dominant hand wields the weapon and off-hand wields the shield is the universal rationale of using a 1hand weapon+shield. So in Pantheon's case, it's just that his left hand happens to be the dominant hand.
It's got nothing to do whether your heart slightly tilts to the left or right when it comes to using the shield. It doesn't matter because if you actually get impaled or slashed open you're dead regardless, especially in medieval times.
Shield is used to block damage, period; not just the damage to the heart. This is especially true to the case with Pantheon whose weapon choice was historically supposed to be used as a part of a infantry phalanx formation, that has a very strong emphasis on defence.