(requested)
Alec tried to keep calm as he saw you with someone other than him. He didn't have a reason to be jealous, he'd chosen to break up with you when he asked Lydia to marry him although that didn't stop his heart from pounding and his palm sweating when he saw you.
Every time he saw you he denied that he had feelings for you reminding himself that he'd chosen to marry Lydia. He found himself daydreaming about you sometimes, unable to focus on any task that was set in front of him.
You walked past him and he couldn't help but ball his fists as you laughed and joked with your newest partner who headed off into the Institute and left you alone. He couldn't take it anymore and moved across the room, dragging you into a side room and spinning you to face him.
"Alec what're you doing!" your answer was his hands roughly pulling you forwards crushing his lips to yours as he let his hands wonder your body.
As soon as he realised what he'd done he pulled away and stormed out of the room not noticing Izzy who'd seen enough to guess what happened.
Alec could see you in your seat, rolling your eyes when you partner muttered something you didn't like. He tried not to let his focus and his mind wonder but he couldn't, part of him was aware of what you were doing all the way though the ceremony.
You thought you'd snuck out quietly, no one had noticed when you left and the corridors were empty so there wasn't anyone so usher you back inside. However, Alec's heart broke a little when you left and he couldn't stand it any longer.
With a muttered apology he jogged down the aisle and burst out of the room and began searching for you. When he found you he was a little out of breath and pulled the clothes you'd been tearfully shoving in an overnight bag out of your hands.
"I'm so sorry (Y/N)." Alec sighed as he pulled you into his arms. "I should have known it's you, you and no one else."
He held you for a while until furious voices could be heard in the corridors and the strong Shadowhunter swallowed nervously. He went to leave but wouldn't let go of you as he wiped tears from your face.
"Promise me you'll be here when I've dealt with them." Alec mumbled as you let him kiss you softly.
"I'll be here." You promised knowing that he'd risked everything by following you out of the wedding.