Macbeth is reported to be a valiant soldier in Act I. The line "Till he unseamed him from the nave to th’chops / And fixed his head upon our...

As your question states, the first time we hear about Macbeth, he is praised as an impressive soldier. When the wounded Captain relates the story of Macbeth's slaying of the rebel Macdonwald, he refers to "brave Macbeth—well he deserves that name" (I.ii.16). King Duncan praises Macbeth as "valiant cousin" and "worthy gentleman" (I.ii.24). However, your question seems to imply that Macbeth's actions don't deserve that descriptor. In fact, given how Macbeth bests Macdonwald (as stated in the...

As your question states, the first time we hear about Macbeth, he is praised as an impressive soldier. When the wounded Captain relates the story of Macbeth's slaying of the rebel Macdonwald, he refers to "brave Macbeth—well he deserves that name" (I.ii.16). King Duncan praises Macbeth as "valiant cousin" and "worthy gentleman" (I.ii.24). However, your question seems to imply that Macbeth's actions don't deserve that descriptor. In fact, given how Macbeth bests Macdonwald (as stated in the lines you quote in your question, from I.ii.22-3), we might be more tempted to call him "barbaric" rather than "valiant."

First of all, remember that "valiant" means "possessing bravery" or doing acts marked by "courage or determination" (Merriam-Webster dictionary). The sheer fact that Macbeth was able to face a commander of the rebel army and best him so handily is, in the context of battle, worthy of being called "valiant."

Secondly, we must consider the time period and circumstances in which Shakespeare's characters (and, really, Shakespeare himself) lived. The historical Macbeth whose life inspired Shakespeare's play ruled in Scotland from 1040–1057. These were the "dark ages," when daily life itself was rough and brutal—and war was even more so.

"Unseaming" your opponent in the violence of hand-to-hand combat was not uncommon. The practice of beheading the commander of an enemy's army goes back to Biblical times (for just one example, see David cutting off Goliath's head); the placing of the head on a pike or battlement was a way to ensure that everyone on both sides saw that the enemy had been bested. Sometimes, even in Shakespeare's day, traitors' heads were placed where people could see them as a way to quell similarly rebellious impulses.

So, given the times and circumstance, as well as the mindset of Macbeth and his contemporaries (and Shakespeare's audience), the actions Macbeth takes on the battlefield are not only commonplace, but deserving (in their minds) of being called "valiant."