Right now we are living in Kali Yuga. And it is said, that in this Yuga, sin is incurred only when the action is performed and not otherwise.
Parashara Smriti's Chapter 1 says in this regard:
- " In the Krita sin is incurred by one who converses (with a sinner) ; in the Treta by one who touches (the sinful man) ; in the
Dvapara by taking the sinner's food ; in the Kali by a (sinful) act
Similarly, the book "Essence of Dharma Sindhu" states:
The fruits of virtuous deeds executed in ten years during Krita Yuga
are equivalent to what a person in TretaYuga would in a year’s
duration; or what ever is earned in a month’s time in Dwapara Yuga;
and what he or she would earn in a day’s time and such is the
magnitude of good deeds in the Kali Yuga!
In fact, even the very thoughts of doing good deeds would fetch
fruitful results, let alone their actual performance.Maha Bhagavata
Kushalaanyasu siddhyanti netaraani kritaaniyat/
(In Kali Yuga, Punya Karmas are deemed to have been performed by mere
Sankalpa and Papa Karmas are deemed to have been perpetrated only
by the actual actions; so said Parashara Maharshi)
And, no, fantasizing about "killing toys" does not seem to be a "sin by mind".
Manu Smriti 12.5. Coveting the property of others, thinking in one’s
heart of what is undesirable, and adherence to false (doctrines), are
the three kinds of (sinful) mental action.
These above are all the sins committed by mind.