Trust.
Lexa knew all about trust. Trust was a lie. It was words from people who wanted something from you. Gustus had always assumed he had Lexa's trust. Perhaps that was why he thought that he could betray her. Perhaps he thought she would not believe the words of a Sky Person, for they surely had not earned the Commander's trust. His word should be worth much more than theirs.
Lexa knew though that Gustus cared for her, and caring, like love was a weakness. A weakness that needed to be stomped out and destroyed. She was the Commander and she had an alliance for clans to control. Weakness would stop her. Trust was simply a symptom of that weakness, and weakness she no longer had.
Love was a weakness. Trust was a weakness. Anyone who said they loved you would just betray you because trust didn't exist. It was a manipulation of emotions to make someone weak. When Clarke had shown her that the drink was no poisoned, Lexa understood the implication straight away. Gustus had not betrayed her because she had not trusted him. But he had defied her orders and he had acted against her.
As life had shown her on more than one occasion it was not reasons that mattered but actions. Reasoning was nothing more than prettily worded excuses for inexcusable actions. Reasons should never be trusted over actions. Gustus' reasons had been fair but his actions had been inspired by weakness and weakness was unacceptable.
Lexa learnt of weakness from Costia. Love and trust had ended in weakness. Love and trust ended in death and betrayal.
There had been no abduction. But Anya was a skilled tracker, and Lexa a good student. Costia had been found with the ice nation and by the time that Lexa traveled back to the woods clan Costia was dead. None of her secrets had been lost. Costia never had the chance.
The next day the Woods clan went to war with the Ice Nation for a lie that would be repeated so often it became the truth. Because love was a weakness and Lexa could not afford to be weak.
When someone said they loved you, what they really meant was that they would eventually betray you. Lexa knew that. She had learnt it the hard way, and she had learnt it well.
But on the eve of battle she was weak. Because she trusted Clarke.
Clarke who spoke righteous words but performed the necessary tasks of war despite her protests. Clarke who instinctively knew what was needed, and was a true leader, unlike her mother. Even if she was still learning. Clarke who willingly chose to use Octavia's love for her brother as a way to buy her silence, but refused more permanent solutions. Clarke who killed to protect as much as she killed for vengeance. Clarke who told the truth, even when she lied, or perhaps she made truth of her words. Clarke who saved and ultimately doomed Anya. Clarke who killed Finn to protect her people in more ways than one. Clarke who trusted Lexa, who let Tondc fall.
Clarke who would stand next to Lexa as they raised the mountain that the Mountain men lived in. Because Lexa trusted her. She trusted Clarke to look after her people, to get them out alive and make sure that the Grounders mostly made it out alive too for good measure. It was an … acceptable thing for Lexa to trust Clarke with.
An acceptable thing, but not the only thing.
(But that wasn't acceptable, that was a weakness)