I do not talk about bitcoin here. Bitcoin is signing transaction perfect. It does not use ring signatures.
Edit:
Alice, Bob and Carol do not want spend their Monero. But hacker Dave wants their money. A ring signature obscures identities because it only proves that a Dave belongs to a group. So Dave ring-signing Alice, Bob Carol and Dave inputs and send XMR to his new stealth addresses.
Edit2:
Monero is same as money on the pavement. So easy to just pick up them from the pavement. Easy money ... but worthless, who can use them?
(Am I wrong ? Why ?)
You're right. What you've pointed out is clearly a gaping hole that allows an attacker to just take everyone's money, and somehow myself, tacotime, smooth, the rest of the Monero core team, andytoshi, gmaxwell, Wladimir v d Laan, Peter Todd, the Monero Research Lab mathematicians / cryptographers, and everyone else that has read the whitepaper and/or looked at the implementation missed this obvious and gaping flaw. Clearly you have the superior intellect and knowledge, I look forward to seeing your published research and PoC code for this flaw soon.
Until then, have a good day.