The first tattoo he got before he was even of age, and a poor job it was, but he would not ask his father for the money for a better one; so he gritted his teeth and glared as the needle bit into him, bleeding dark ink beneath his skin.
Thorin called him an idiot, and not even that politely. Dwalin did not give him a friendly punch in return for the insult, as he would have before; his hand hurt still from the inking, true, but he also knew, now, of what deeds those hands were capable. Thorin raised his eyebrows at his friend's new-found restraint, but made no comment, and let him be.
Dwalin took to sparring on his own; pounding his fists and his fury into leather bags, and then into walls and trees, things that could bear the brunt of his fearsome strength - or if they could not, would be no loss themselves.
After a few days Thorin tracked him down. He handed Dwalin a flask of ale and a honey cake, and pointed to the nearest rock. 'Sit.' Dwalin sat, sullenly, not because Thorin was his prince but because he was his friend. He swallowed some ale and half the honey cake, passing the remainder to Thorin. The prince frowned over Dwalin's bloodied knuckles.
'You need to tend to these cuts, my friend.'
Dwalin scowled, and drew his hand back, folding his arms tight over his chest. 'What's the point?'
The younger dwarf shot him a piercing glance. 'Would you treat an axe or any other weapon with so little care? A tool uncared for - '
'- loses its edge, aye, I know. All right.' Dwalin snorted and shook his head, taking another swig from the flask and passing it to Thorin, who drank in his turn. The light was fading around them and lamps were being lit in the windows of the mountain. They sat in silence for a time.
'It was an accident,' Dwalin said softly, at last. He did not look at his companion, staring instead at the word tattooed on the back of his right hand, tracing it with the fingers of his left, but he could feel Thorin's gaze upon him.
'Was it?' There was no judgement in Thorin's voice; it was a simple question, and Dwalin was grateful for it.
'You were not unprovoked,' Thorin said, 'and you were acting in the defense of someone who could not defend themselves.'
'Aye. But he could not have truly harmed me, knives or no. I was the better fighter... And I lost control, and his neck broke in my hands like a wafer of cram.'
Thorin was frowning. 'Your brother is a great warrior in the royal retinue and you yourself will begin training for the guard in a little over a year. We are not a people made for peace, Dwalin.'
'You do not understand.' Dwalin rose to his feet, crossing his arms again and glaring down at Thorin. 'Do you not know the tales of the horsemasters, who go mad with a battle lust that will see them kill their dearest friends when the fell mood is on them? It is the same with me. I felt it, Thorin.' And for the first time, for a single moment, Thorin saw fear on Dwalin's face. 'I still feel it; like a beast sleeping only lightly.' And he laughed, but there was no joy in the sound.
'Do you think I do not know that feeling?' Thorin said quietly. He rose also, dark hair and dark tunic all but invisible against the night, but his eyes shone. 'Not for blood and battle, but for gold?'
The moon was not yet risen and the air was growing chill; behind them the mountain waited, the lamps shining like captive stars. Thorin sighed and clapped a hand on Dwalin's shoulder. 'Come, amradûn,' he said, and from his lips the word was as gentle as it could be made. 'Let us take our beasts, and go home.'
amradûn - 'death-man', the closest Khuzdûl I could find to 'killer'.