Syria and sarin gas have been in the news lately. I wrote about Syria years ago here. It is unfortunate that more civilians had to die before the United States reacted with a cruise missile strike on the airport from which the chemical weapon strike originated. This specific action by the United States, without taking into consideration the "big picture" of our Syria plan, was certainly justified. In fact, President Obama's decision to do nothing, even after drawing a line in the sand, was morally unjustified.
Today, news broke that the United States used the largest convention bomb ever created in Afghanistan on a target affiliated with ISIS. Some have called attention to the two and labeled the United States as hypocritical. Here is why that accusation shows a lack of or flawed understanding of Just War tradition.
Whether a weapon is outlawed or not is based on the concept of noncombatant immunity. Sarin gas is outlawed not (only) because of the horrible way in which it kills. It is because there is no way to control whom it kills. The wind carries it wherever it wants. Sarin gas is an indiscriminate weapon incapable of killing combatants without harming civilians.
Other weapons in this category are biological weapons, whose spread is left to the laws of epidemiology and chance, and nuclear weapons, whose initial blast target only combatants (granted, it would have to be one big battlefield), but whose secondary effects (fallout, radiation in the atmosphere, climate effects) are not targetable.
Conventional bombs do not have these problems (although they have other problems). If a conventional bomb is properly aimed with good intelligence, it is capable of discriminating between combatant and civilian. This doesn't mean every use of a conventional weapon is just. A conventional bomb dropped on a hospital obviously isn't. Or one dropped by a poorly trained soldier. Or one dropped with no or bad intelligence.
Chemical/Biological/Nuclear weapons are inherently unjust because of their indiscriminate nature.
Conventional bombs are not inherently unjust (but are only just when many other criteria are met).