Using 'guiltily' adds emotional depth, indicating inner conflict or shame. He glanced guiltily at his friend after breaking the news.
Pro Tip 2/3
Nonverbal Cues
'Guiltily' often refers to physical expressions of guilt, like avoiding eye contact. She touched the broken vase and looked up guiltily.
Pro Tip 3/3
Subtle Admission
Saying something 'guiltily' can imply a subtle, reluctant confession without direct acknowledgment. I might have accidentally seen your letter, he said guiltily.