I shall presume a situation where the Lizard-folk do not speak the common tongue - or do so very poorly.
I shall also presume a situation where these are likely to retain a stiff upper lip and pose a 'tough exterior to threats' as may be in line with their culture.
Both of these serve as obstacles to diplomacy - and both of these can serve to foster serious misunderstandings between races that ill-understand each other and are liable to intentionally or accidentally over-blow incidents.
Enforcing But Never (or rarely) Crossing One's Borders
A good-aligned tribe is a lot less likely to take actions to expand their borders, building of settlements in shows of physical occupation and leading excursions outside their territories for the purpose of oppressing those lands' inhabitants and claiming possession of such lands.
A good-aligned tribe is more likely to take action to enforce its own territories, intimidating those who straggle onto their turf, perhaps also attacking. They are more likely to be fierce defenders where their ferocity can border on brutality, but not for brutality's sake.
A history of conflict with (for example) humans will naturally up the intensity of hostilities but they are more likely to neutralize the threat that trespassers pose rather than killing them outright.
Cultural Manifestations of Alignment
The culture of the lizard-folk can occasionally shine through - particularly with regard to signs of restraint - such as not delivering a fatal blow when they have opportunity to do so, or shying away from conflict in neutral territories.
Perhaps their hunting parties might include a support shaman who specializes in spells intended to dis-empower foes. Perhaps spells that suggest defensive rather than offensive purposes (entangle, slow, summon X creature).
Diplomacy From a Position of Strength
I would imagine that they would be rather difficult to engage in diplomatic conversation considering their previous contacts with outsiders of the PCs' sorts. Some players would not really think much of going in with axes and magic missiles flailing.
This is why you will want to consider throwing in instances where the PCs face unfavorable odds (mass ambush). They are less likely to get fighty in situations where they are unlikely to prevail.
Citizens, especially children, have a tendency of wandering further than their more entrenched elders. This applies to most species and it just might set up an interpreter relationship if one of the lizard-kind happens to have less lousy common than the rest.
Cruel Town Cohorts
Perhaps one way of drumming things into the heads of the PCs, indirectly is to gain them an opportunity to participate in a lizard-repelling (hunting) expedition "in the name of ensuring the safety of travelers in the area".
This, of course, extends to clashing with a river-side sheltered enclave with a very small community little more numerous than the raiding party and of mixed levels of ability - including a good half that are bumbling civilians with improvised weapons.
Heart-string tugging moments such as the involvement of familial connections may be inserted but of greater interest would be the wanton and contemptuous brutality of the town mercenary party in slaying and looting. If it happens to be a longer campaign element with a very significant lizard-folk presence in the area - this could provide several opportunities for the PCs to realize that they might be committing an error.
Clues - Options, not Hooks
Of course be prepared for all kinds of play preferences... maybe they completely miss it in favor of lizard-slaying campaigns - or they actually have an evil bone or three in them - but perhaps the PCs will never know their error - having contributed to genocide... perhaps only revealed when another wiser or more influential entity enters the area.