If a beast slain in the Jyotishtoma rite will itself go to heaven
This is from the observer perspective. Beast itself doesn't get sacrificed for the sake of attaining heaven. Like how soldiers don't get martyred themselves just to go to heaven.
[ Add on reference: Martyr concept in Hinduism ]
That means, beast (or a soldier) would be living normally. Upon its master's (country's) requirement, it will be sacrificed (martyred). This falls under action out of duty, where one acts without desire of result. Such sacrifice is termed as SAtvika:
BG 18.23 - The regular obligatory action which is performed without attachment and without likes or dislikes by one who does not hanker for rewards, that is said to be born of sattva
The actions performed under sattva has the heavenly happiness:
BG 14.9 - ... sattva attaches one to happiness, ...
BG 14.14 - When an embodied one undergoes death while mainly sattva is predominant, then they attain the taint-less plane of knower of the highest
BG 14.18 - Those with sattva go higher [regions] ...
Why then does not the sacrificer forthwith offer his own father?
Because father will not attain heaven.
What is the purpose of offering father?
- Is it in their tribe or local's constitution or scripture -- NO
- Will father be useful for eating -- NO
- Is there any other purpose for sacrificing father -- NO
So here the sole purpose is to send father to heaven. Since it's a fruitive action, it becomes RAjasika in nature.
BG 18.24 - But that action is said to be born of rajas, which is done by one desirous of results or by one who is egotistic, and which is highly strenuous.
Actions performed under RAjasika would result in sin followed by sorrow.
Refer Sin according to Hinduism.